feat: implement engine kind detection for default context in context ls

[ilopezluna]

 docker model context ls
NAME     HOST                    DESCRIPTION                     CURRENT  
default  Docker Desktop          Model Runner on Docker Desktop           
debug    http://localhost:13434                                  *       

[gemini-code-assist]

Code Review

This pull request implements dynamic engine kind detection for the "default" Docker context to provide more accurate host and description information in the CLI. However, the current implementation hardcodes connection schemes and ports, failing to account for TLS configurations. Additionally, the detection logic is inefficiently called within loops, potentially causing significant delays, and it lacks handling for specific WSL2 edge cases where Moby-based runners are used.

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• In resolveDefaultContext, consider adding a default branch in the switch kind to ensure host is always initialized sensibly if new ModelRunnerEngineKind values are introduced in the future.
• The host/port construction logic in resolveDefaultContext duplicates logic from the runtime detection path; it might be worth centralizing this mapping (kind + TLS → URL) in a shared helper to avoid future drift between what is displayed and what is actually used.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `resolveDefaultContext`, consider adding a `default` branch in the `switch kind` to ensure `host` is always initialized sensibly if new `ModelRunnerEngineKind` values are introduced in the future.
- The host/port construction logic in `resolveDefaultContext` duplicates logic from the runtime detection path; it might be worth centralizing this mapping (kind + TLS → URL) in a shared helper to avoid future drift between what is displayed and what is actually used.

## Individual Comments

### Comment 1
<location path="cmd/cli/commands/context.go" line_range="94-101" />
<code_context>
+	detectCtx, cancel := context.WithTimeout(ctx, defaultContextDetectTimeout)
+	defer cancel()
+
+	kind := desktop.DetectEngineKind(detectCtx, dockerCLI)
+	description = "Model Runner on " + kind.String()
+
+	// Check whether TLS is enabled so the displayed scheme and port match
+	// what DetectContext would actually use at runtime.
+	useTLS := os.Getenv("MODEL_RUNNER_TLS") == "true"
+
+	switch kind {
+	case types.ModelRunnerEngineKindDesktop:
+		host = kind.String()
</code_context>
<issue_to_address>
**issue:** Handle unexpected/unknown engine kinds to avoid returning an empty host string

If `DetectEngineKind` ever returns a new/unknown `ModelRunnerEngineKind` (e.g., after future enum extensions), this `switch` will leave `host` empty while still setting a non-empty `description`. Consider adding a `default` case that falls back to the generic `(auto-detect)` host (and possibly description) to keep the output consistent and user-friendly for unhandled kinds.
</issue_to_address>

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