Remove connection_pool args and extend chat processing

[creatorrr]

trafficstars

Summary

• drop unused connection_pool parameters from docs and chat endpoints
• store all response choices in session history
• add streaming support
• handle truncation, datetime serialization and tool formatting in render

Testing

• ruff format --diff --quiet .
• ruff check agents-api/agents_api/routers/sessions/chat.py agents-api/agents_api/routers/docs/create_doc.py agents-api/agents_api/routers/docs/search_docs.py agents-api/agents_api/routers/responses/create_response.py agents-api/agents_api/routers/sessions/render.py