fix: separate document and query prefixes for embedding models

[roomote-v0]

Related GitHub Issue

Closes: #11707

Description

This PR attempts to address Issue #11707 by separating document (indexing) and query (search) prefixes for embedding models. Feedback and guidance are welcome.

Problem: The queryPrefix in EmbeddingModelProfile was applied to all texts indiscriminately -- both during indexing (scanner/file-watcher) and during search queries. Most modern embedding models (including nomic-embed-code) require different instructions for indexing vs querying, or at minimum: no prefix for indexing and a specific prefix for querying.

Solution:

• Added EmbeddingPurpose type ("index" | "query") to @roo-code/types
• Added optional documentPrefix field to EmbeddingModelProfile for indexing/document embedding
• Added getModelDocumentPrefix() and getModelPrefixForPurpose() helpers
• Added optional purpose parameter to IEmbedder.createEmbeddings():
  • "index" -> uses documentPrefix (for document indexing)
  • "query" or undefined -> uses queryPrefix (backward compatible)
• Updated all 8 embedder implementations to accept and pass through the purpose parameter
• Scanner and file-watcher now pass "index" purpose
• Search service now passes "query" purpose
• nomic-embed-code profiles: queryPrefix kept for search, documentPrefix intentionally omitted (undefined) since nomic does not require a prefix for document indexing

Backward compatibility: Models without documentPrefix continue to work exactly as before. The purpose parameter is optional and defaults to queryPrefix behavior when omitted.

Test Procedure

• Added 14 new tests in embeddingModels.spec.ts covering getModelQueryPrefix, getModelDocumentPrefix, and getModelPrefixForPurpose
• Updated 5 existing test assertions in gemini, mistral, and vercel-ai-gateway specs for the new purpose parameter passthrough
• All 407 tests pass (26 embeddingModels + 174 embedders + 207 processors/services)
• Lint and type checks pass across the entire monorepo

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes.
• Documentation Impact: No documentation updates required -- this is an internal API change.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

This change is fully backward compatible. Existing callers that do not pass a purpose will continue to use queryPrefix as before.

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