Persist chat history metadata in OpenSearch for multi-instance backends #1073

[kowshik24]

Issue: #1073

Summary:

• Reworked chat metadata persistence from local JSON-file storage to an OpenSearch-backed service, including a dedicated chat_conversation_metadata index schema and async CRUD operations, with in-memory fallback retained for resiliency when OpenSearch is unavailable.
• Updated startup index initialization so the backend automatically creates the chat conversation metadata index during boot, enabling shared metadata across horizontally scaled instances.
• Updated chat history paths to await async metadata retrieval and aligned agent helpers with async conversation metadata access so history reads/writes flow through centralized storage logic.
• Added unit tests for OpenSearch-backed storage/retrieval behavior and fallback behavior when OpenSearch is unavailable.

Testing:
- ⚠️ pytest -q tests/unit/test_conversation_persistence_service.py (fails in this environment due to missing pytest_asyncio dependency).
- ✅ python -m compileall src tests/unit/test_conversation_persistence_service.py