Add IHostedConversationClient abstraction for hosted conversation lifecycle management

docs/HostedConversation-ProviderMapping.md

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+# IHostedConversationClient — Provider Mapping Report
+
+## Overview
+
+`IHostedConversationClient` is an abstraction for managing server-side (hosted) conversation state across AI providers. It provides a common interface for creating, retrieving, deleting, and managing messages within persistent conversations, decoupling application code from provider-specific conversation/thread/session APIs. Each provider maps these operations to its native primitives, with escape hatches (`RawRepresentation`, `RawRepresentationFactory`, `AdditionalProperties`) for accessing provider-specific features.
+
+## Interface Operations
+
+| Operation | Description | Return Type |
+|-----------|-------------|-------------|
+| `CreateAsync` | Creates a new hosted conversation | `HostedConversation` |
+| `GetAsync` | Retrieves conversation by ID | `HostedConversation` |
+| `DeleteAsync` | Deletes a conversation | `void` (Task) |
+| `AddMessagesAsync` | Adds messages to a conversation | `void` (Task) |
+| `GetMessagesAsync` | Lists messages in a conversation | `IAsyncEnumerable<ChatMessage>` |
+
+## Provider Mapping
+
+### OpenAI (Implemented)
+
+- Maps to `ConversationClient` in `OpenAI.Conversations` namespace
+- Full CRUD support via protocol-level APIs
+- `RawRepresentation` set to `ClientResult` objects
+- ConversationId integrates with `ChatOptions.ConversationId` for inference via `OpenAIResponsesChatClient`
+- Metadata limited to 16 key-value pairs (max 64 char keys, 512 char values)
+
+### Azure AI Foundry
+
+- Maps to Thread/Message APIs in Agent Service SDK
+- `CreateAsync` → `threads.create()`
+- `GetAsync` → `threads.get()`
+- `DeleteAsync` → `threads.delete()`
+- `AddMessagesAsync` → `messages.create()` (one per message)
+- `GetMessagesAsync` → `messages.list()`
+- **Gaps**: Thread model includes Run/Agent concepts not in our abstraction; use `AdditionalProperties` for agent-specific metadata
+
+### AWS Bedrock
+
+- Maps to Session Management APIs
+- `CreateAsync` → `CreateSession` with optional encryption/metadata
+- `GetAsync` → `GetSession`
+- `DeleteAsync` → `DeleteSession`
+- `AddMessagesAsync` → `PutInvocationStep` (different item model)
+- `GetMessagesAsync` → `GetInvocationSteps` (requires translation)
+- **Gaps**: Session status (ACTIVE/EXPIRED/ENDED) not in abstraction; encryption config is provider-specific; use `AdditionalProperties` or `RawRepresentationFactory`
+
+### Google Gemini
+
+- Maps to Interactions API
+- `CreateAsync` → `interactions.create()` (creates an interaction, not a "conversation" per se)
+- `GetAsync` → `interactions.get()`
+- `DeleteAsync` → `interactions.delete()`
+- `AddMessagesAsync` → No direct equivalent; use `interactions.create()` with `previous_interaction_id` chain
+- `GetMessagesAsync` → `interactions.get().outputs` (retrieves outputs, not full message history)
+- **Gaps**: Interactions are individual turns, not conversation containers. AddMessages requires creating new interactions chained via `previous_interaction_id`. Provider adapter would need to manage this mapping.
+
+### Anthropic
+
+- **No native conversation CRUD API** — requires local adapter
+- Server-side features that CAN assist:
+  - **Prompt Caching** (`cache_control`): Stores KV cache of message prefixes (5min/1hr TTL). Adapter should auto-apply cache breakpoints.
+  - **Context Compaction** (beta): Server-side summarization when conversations exceed token threshold
+  - **Files API** (beta): Store documents server-side for reference across requests
+  - **Containers** (beta): Server-side execution state with reusable IDs
+- Implementation approach: `LocalHostedConversationClient<TStore>` using local storage (in-memory, SQLite, Redis) with automatic prompt caching optimization
+- **Gaps**: All operations are simulated client-side. No server-side conversation persistence.
+
+### Ollama / Local Models
+
+- **No server-side state** at all
+- Implementation: Same local adapter pattern as Anthropic but without prompt caching optimization
+- **Gaps**: Same as Anthropic — entirely client-side simulation
+
+## Escape Hatches for Provider-Specific Features
+
+### RawRepresentation
+
+Every `HostedConversation` response carries `RawRepresentation` (the underlying provider object). This gives access to 100% of provider functionality:
+
+```csharp
+var conversation = await client.CreateAsync();
+var openAIResult = (ClientResult)conversation.RawRepresentation; // Access any OpenAI-specific data
+```
+
+### RawRepresentationFactory
+
+`HostedConversationCreationOptions.RawRepresentationFactory` allows passing provider-specific creation options:
+
+```csharp
+var options = new HostedConversationCreationOptions
+{
+    RawRepresentationFactory = client => new ConversationCreationOptions
+    {
+        // Any provider-specific settings
+    }
+};
+```
+
+### AdditionalProperties
+
+`HostedConversation.AdditionalProperties` and `HostedConversationCreationOptions.AdditionalProperties` carry provider-specific data that doesn't fit the common abstraction.
+
+## Feature Coverage Matrix
+
+| Feature | OpenAI | Azure | Bedrock | Gemini | Anthropic | Ollama |
+|---------|--------|-------|---------|--------|-----------|--------|
+| Create | ✅ Native | ✅ Native | ✅ Native | ✅ Native | ⚠️ Local | ⚠️ Local |
+| Get | ✅ Native | ✅ Native | ✅ Native | ✅ Native | ⚠️ Local | ⚠️ Local |
+| Delete | ✅ Native | ✅ Native | ✅ Native | ✅ Native | ⚠️ Local | ⚠️ Local |
+| AddMessages | ✅ Native | ✅ Native | ⚠️ Translated | ⚠️ Chained | ⚠️ Local | ⚠️ Local |
+| GetMessages | ✅ Native | ✅ Native | ⚠️ Translated | ⚠️ Partial | ⚠️ Local | ⚠️ Local |
+| Metadata | ✅ 16 KV | ✅ | ✅ | ✅ | ⚠️ Local | ⚠️ Local |
+| RawRepresentation | ✅ ClientResult | ✅ AgentThread | ✅ Session | ✅ Interaction | N/A | N/A |
+
+Legend: ✅ = Direct mapping, ⚠️ = Requires translation/local adapter

src/Libraries/Microsoft.Extensions.AI.Abstractions/HostedConversation/DelegatingHostedConversationClient.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Extensions.AI;
+
+/// <summary>
+/// Provides an optional base class for an <see cref="IHostedConversationClient"/> that passes through calls to another instance.
+/// </summary>
+/// <remarks>
+/// This is recommended as a base type when building clients that can be chained around an underlying <see cref="IHostedConversationClient"/>.
+/// The default implementation simply passes each call to the inner client instance.
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AIHostedConversation, UrlFormat = DiagnosticIds.UrlFormat)]
+public class DelegatingHostedConversationClient : IHostedConversationClient
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="DelegatingHostedConversationClient"/> class.
+    /// </summary>
+    /// <param name="innerClient">The wrapped client instance.</param>
+    /// <exception cref="ArgumentNullException"><paramref name="innerClient"/> is <see langword="null"/>.</exception>
+    protected DelegatingHostedConversationClient(IHostedConversationClient innerClient)
+    {
+        InnerClient = Throw.IfNull(innerClient);
+    }
+
+    /// <inheritdoc />
+    public void Dispose()
+    {
+        Dispose(disposing: true);
+        GC.SuppressFinalize(this);
+    }
+
+    /// <summary>Gets the inner <see cref="IHostedConversationClient" />.</summary>
+    protected IHostedConversationClient InnerClient { get; }
+
+    /// <inheritdoc />
+    public virtual Task<HostedConversation> CreateAsync(
+        HostedConversationCreationOptions? options = null,
+        CancellationToken cancellationToken = default) =>
+        InnerClient.CreateAsync(options, cancellationToken);
+
+    /// <inheritdoc />
+    public virtual Task<HostedConversation> GetAsync(
+        string conversationId,
+        CancellationToken cancellationToken = default) =>
+        InnerClient.GetAsync(conversationId, cancellationToken);
+
+    /// <inheritdoc />
+    public virtual Task DeleteAsync(
+        string conversationId,
+        CancellationToken cancellationToken = default) =>
+        InnerClient.DeleteAsync(conversationId, cancellationToken);
+
+    /// <inheritdoc />
+    public virtual Task AddMessagesAsync(
+        string conversationId,
+        IEnumerable<ChatMessage> messages,
+        CancellationToken cancellationToken = default) =>
+        InnerClient.AddMessagesAsync(conversationId, messages, cancellationToken);
+
+    /// <inheritdoc />
+    public virtual IAsyncEnumerable<ChatMessage> GetMessagesAsync(
+        string conversationId,
+        CancellationToken cancellationToken = default) =>
+        InnerClient.GetMessagesAsync(conversationId, cancellationToken);
+
+    /// <inheritdoc />
+    public virtual object? GetService(Type serviceType, object? serviceKey = null)
+    {
+        _ = Throw.IfNull(serviceType);
+
+        // If the key is non-null, we don't know what it means so pass through to the inner service.
+        return
+            serviceKey is null && serviceType.IsInstanceOfType(this) ? this :
+            InnerClient.GetService(serviceType, serviceKey);
+    }
+
+    /// <summary>Provides a mechanism for releasing unmanaged resources.</summary>
+    /// <param name="disposing"><see langword="true"/> if being called from <see cref="Dispose()"/>; otherwise, <see langword="false"/>.</param>
+    protected virtual void Dispose(bool disposing)
+    {
+        if (disposing)
+        {
+            InnerClient.Dispose();
+        }
+    }
+}

src/Libraries/Microsoft.Extensions.AI.Abstractions/HostedConversation/HostedConversation.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Extensions.AI;
+
+/// <summary>Represents a hosted conversation.</summary>
+[Experimental(DiagnosticIds.Experiments.AIHostedConversation, UrlFormat = DiagnosticIds.UrlFormat)]
+public class HostedConversation
+{
+    /// <summary>Gets or sets the conversation identifier.</summary>
+    public string? ConversationId { get; set; }
+
+    /// <summary>Gets or sets the creation timestamp.</summary>
+    public DateTimeOffset? CreatedAt { get; set; }
+
+    /// <summary>Gets or sets metadata associated with the conversation.</summary>
+    public AdditionalPropertiesDictionary<string>? Metadata { get; set; }
+
+    /// <summary>Gets or sets the raw representation of the conversation from the underlying provider.</summary>
+    /// <remarks>
+    /// If a <see cref="HostedConversation"/> is created to represent some underlying object from another object
+    /// model, this property can be used to store that original object. This can be useful for debugging or
+    /// for enabling a consumer to access the underlying object model if needed.
+    /// </remarks>
+    [JsonIgnore]
+    public object? RawRepresentation { get; set; }
+
+    /// <summary>Gets or sets any additional properties associated with the conversation.</summary>
+    public AdditionalPropertiesDictionary? AdditionalProperties { get; set; }
+}

src/Libraries/Microsoft.Extensions.AI.Abstractions/HostedConversation/HostedConversationClientExtensions.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Extensions.AI;
+
+/// <summary>Provides a collection of static methods for extending <see cref="IHostedConversationClient"/> instances.</summary>
+[Experimental(DiagnosticIds.Experiments.AIHostedConversation, UrlFormat = DiagnosticIds.UrlFormat)]
+public static class HostedConversationClientExtensions
+{
+    /// <summary>Asks the <see cref="IHostedConversationClient"/> for an object of type <typeparamref name="TService"/>.</summary>
+    /// <typeparam name="TService">The type of the object to be retrieved.</typeparam>
+    /// <param name="client">The client.</param>
+    /// <param name="serviceKey">An optional key that can be used to help identify the target service.</param>
+    /// <returns>The found object, otherwise <see langword="null"/>.</returns>
+    /// <exception cref="ArgumentNullException"><paramref name="client"/> is <see langword="null"/>.</exception>
+    /// <remarks>
+    /// The purpose of this method is to allow for the retrieval of strongly typed services that may be provided by the <see cref="IHostedConversationClient"/>,
+    /// including itself or any services it might be wrapping.
+    /// </remarks>
+    public static TService? GetService<TService>(this IHostedConversationClient client, object? serviceKey = null)
+    {
+        _ = Throw.IfNull(client);
+
+        return client.GetService(typeof(TService), serviceKey) is TService service ? service : default;
+    }
+
+    /// <summary>
+    /// Asks the <see cref="IHostedConversationClient"/> for an object of the specified type <paramref name="serviceType"/>
+    /// and throws an exception if one isn't available.
+    /// </summary>
+    /// <param name="client">The client.</param>
+    /// <param name="serviceType">The type of object being requested.</param>
+    /// <param name="serviceKey">An optional key that can be used to help identify the target service.</param>
+    /// <returns>The found object.</returns>
+    /// <exception cref="ArgumentNullException"><paramref name="client"/> is <see langword="null"/>.</exception>
+    /// <exception cref="ArgumentNullException"><paramref name="serviceType"/> is <see langword="null"/>.</exception>
+    /// <exception cref="InvalidOperationException">No service of the requested type for the specified key is available.</exception>
+    /// <remarks>
+    /// The purpose of this method is to allow for the retrieval of services that are required to be provided by the <see cref="IHostedConversationClient"/>,
+    /// including itself or any services it might be wrapping.
+    /// </remarks>
+    public static object GetRequiredService(this IHostedConversationClient client, Type serviceType, object? serviceKey = null)
+    {
+        _ = Throw.IfNull(client);
+        _ = Throw.IfNull(serviceType);
+
+        return
+            client.GetService(serviceType, serviceKey) ??
+            throw Throw.CreateMissingServiceException(serviceType, serviceKey);
+    }
+
+    /// <summary>
+    /// Asks the <see cref="IHostedConversationClient"/> for an object of type <typeparamref name="TService"/>
+    /// and throws an exception if one isn't available.
+    /// </summary>
+    /// <typeparam name="TService">The type of the object to be retrieved.</typeparam>
+    /// <param name="client">The client.</param>
+    /// <param name="serviceKey">An optional key that can be used to help identify the target service.</param>
+    /// <returns>The found object.</returns>
+    /// <exception cref="ArgumentNullException"><paramref name="client"/> is <see langword="null"/>.</exception>
+    /// <exception cref="InvalidOperationException">No service of the requested type for the specified key is available.</exception>
+    /// <remarks>
+    /// The purpose of this method is to allow for the retrieval of strongly typed services that are required to be provided by the <see cref="IHostedConversationClient"/>,
+    /// including itself or any services it might be wrapping.
+    /// </remarks>
+    public static TService GetRequiredService<TService>(this IHostedConversationClient client, object? serviceKey = null)
+    {
+        _ = Throw.IfNull(client);
+
+        if (client.GetService(typeof(TService), serviceKey) is not TService service)
+        {
+            throw Throw.CreateMissingServiceException(typeof(TService), serviceKey);
+        }
+
+        return service;
+    }
+}

src/Libraries/Microsoft.Extensions.AI.Abstractions/HostedConversation/HostedConversationClientMetadata.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Extensions.AI;
+
+/// <summary>Provides metadata about an <see cref="IHostedConversationClient"/>.</summary>
+[Experimental(DiagnosticIds.Experiments.AIHostedConversation, UrlFormat = DiagnosticIds.UrlFormat)]
+public class HostedConversationClientMetadata
+{
+    /// <summary>Initializes a new instance of the <see cref="HostedConversationClientMetadata"/> class.</summary>
+    /// <param name="providerName">
+    /// The name of the hosted conversation provider, if applicable. Where possible, this should map to the
+    /// appropriate name defined in the OpenTelemetry Semantic Conventions for Generative AI systems.
+    /// </param>
+    /// <param name="providerUri">The URL for accessing the hosted conversation provider, if applicable.</param>
+    public HostedConversationClientMetadata(string? providerName = null, Uri? providerUri = null)
+    {
+        ProviderName = providerName;
+        ProviderUri = providerUri;
+    }
+
+    /// <summary>Gets the name of the hosted conversation provider.</summary>
+    /// <remarks>
+    /// Where possible, this maps to the appropriate name defined in the
+    /// OpenTelemetry Semantic Conventions for Generative AI systems.
+    /// </remarks>
+    public string? ProviderName { get; }
+
+    /// <summary>Gets the URL for accessing the hosted conversation provider.</summary>
+    public Uri? ProviderUri { get; }
+}