feat: add typed cross-collection face search

[imronreviady]

Summary

• Expose the existing public POST /search endpoint through the faces resource.
• Parse typed cross-collection matches, collectionsSearched and skippedCollections (id, name, reason).
• Surface HTTP 409 EMBEDDING_PROFILE_MISMATCH through the SDK's existing typed API-error path.
• Cover the request contract, partial results and mismatch handling in tests.

Verification

• pytest: 50 passed; mypy passed.

Part of versioned-embeddings; companion backend PR: https://github.com/LiveXFace/backend/pull/23
No SDK package release or version tag is created by this PR.