[mattapperson]

Description

Fixes #587

The chat model built the request body as { ...this.getArgs(options), ...restOpenrouterOptions }, so a call-time providerOptions.openrouter.provider replaced the configured settings.provider / extraBody.provider outright. Configured keys such as allow_fallbacks and sort were dropped. The completion model had the same problem with extraBody.provider.

Changes:

• Shared helper src/utils/merge-provider-routing.ts, mergeProviderRouting(configured, runtime):
  • shallow merges plain-object sources (prototype Object.prototype or null), later sources winning; nested arrays/objects (order, only, ignore, max_price) are replaced, not merged
  • ignores non-plain-object configured values (undefined, null, arrays, primitives, class instances)
  • an explicit runtime provider: null clears routing entirely (returns undefined), keeping the escape hatch that existed on main for chat/completion
  • returns undefined when nothing applies
• Chat doGenerate / doStream: settings.provider < factory extraBody.provider < model extraBody.provider < providerOptions.openrouter.provider.
• Completion doGenerate / doStream: factory extraBody.provider < model extraBody.provider < runtime. Completion settings have no provider field.
• Chat, completion, image and embedding always assign provider to the helper result, so a non-object provider coming from extraBody (e.g. extraBody: { provider: null }) never leaks into the body. undefined is dropped by JSON serialization.
• Image (fix(image): merge runtime provider options with configured routing preferences #551) and embedding (fix(embedding): forward providerOptions.openrouter in embed/embedMany #632) now use the shared helper; the local copy in src/embedding/index.ts is removed.
• README gets a Provider Routing section documenting precedence, shallow-merge semantics, and how to drop configured keys (null clears everything).

Behavior changes (see changeset, minor):

• Chat/completion: a runtime provider object now merges with configured routing instead of replacing it. Runtime null still clears routing.
• Image/embedding: runtime provider: null previously was ignored (configured routing kept); it now clears routing, consistent with chat/completion. Image no longer forwards non-object extraBody.provider values.

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

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