[mattapperson]

Description

Related to #616.

The embedding model ignored call-time providerOptions.openrouter. doEmbed built the request body from settings and extraBody only, so embed / embedMany calls like providerOptions: { openrouter: { provider: { only: ['x'] } } } sent no provider at all. Chat and completion already forward these options.

Changes:

• doEmbed now takes EmbeddingModelV4CallOptions, reads providerOptions.openrouter, and spreads it into the body after settings / extraBody, the same way chat and completion do. Any field the /embeddings endpoint accepts (provider, user, dimensions, input_type, session_id, trace, ...) passes through.
• provider routing is shallow merged with precedence settings.provider < config.extraBody.provider < settings.extraBody.provider < providerOptions.openrouter.provider. Null, array and primitive values are ignored. One behavior change: an extraBody.provider used to replace settings.provider; now the two merge.
• README: new "Embedding Provider Options" section.

Relationship to #628

#628 (still open) adds a shared src/utils/merge-provider-routing.ts helper with the same semantics for chat, completion and image. This PR doesn't depend on #628 and doesn't touch any of its files. It uses a small local mergeProviderRouting in src/embedding/index.ts, with a TODO(#628) to swap in the shared helper once that PR merges. The two can merge in either order without conflicts.

Tests

• Unit tests (src/embedding/index.test.ts) run real embed / embedMany against a mocked fetch and check the request body for: runtime-only provider, merging with settings.provider, the full precedence chain, ignoring a null runtime provider, passthrough of other options (user, dimensions, input_type, session_id), an unchanged body when no providerOptions are given, and provider left out when nothing sets it. 6 of these fail on main.
• E2E regression test e2e/issues/issue-616-embedding-provider-options.test.ts: a live run against OpenRouter passes here. On main, 3 of its 4 cases fail: an unknown provider.only isn't rejected, and dimensions: 256 is ignored.

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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