Enable text-only vector search

[creatorrr]

trafficstars

Summary

• make vector field optional in docs search models
• embed raw text automatically when vector search/hybrid search receives only text
• adjust router logic and add new tests

Testing

• ruff format agents-api/agents_api/routers/docs/search_docs.py agents-api/tests/test_docs_routes.py agents-api/agents_api/autogen/Docs.py
• ruff check agents-api/agents_api/routers/docs/search_docs.py agents-api/tests/test_docs_routes.py agents-api/agents_api/autogen/Docs.py
• pyright agents-api (fails: Import "ward" could not be resolved)