Mongodb memory session

[DamyanBG]

✨ Add MongoDB-Based Async Session Store

This PR adds an asynchronous session store implementation using MongoDB and the new async API available in pymongo >=4.13. The issue is #1362.

🔧 Summary

• Implements MongoDBSession, conforming to the existing SessionABC interface.
• Fully asynchronous using pymongo’s async client (AsyncMongoClient).
• Schema-less but initializes collections and indexes if they do not already exist.
• Provides full support for:
  • Session initialization
  • Adding, retrieving, and removing session messages
  • Clearing session data
  • Context-managed resource cleanup via __aenter__ / __aexit__

📦 Dependencies

This implementation relies on:

pymongo>=4.13  # or compatible async version

Users can install this with:

pip install 'openai-agents[pymongo]'

✅ Next Steps

• I will add unit tests once the overall implementation pattern is validated.
• I plan to align with the other session store PRs (PostgreSQL, Redis, SQLAlchemy) once they are finalized and merged to ensure consistency in naming, structure, and behavior.

🧠 Notes

• Indexes are created on session_id for agent_sessions, and on session_id + created_at for agent_messages.
• A _initialized flag ensures index creation runs only once per session instance.
• The code gracefully skips invalid JSON entries in the database.
• I’ve used json.dumps() and json.loads() for storing/retrieving items, matching the format used by other stores.

[seratch]

Thanks for sending this. We cannot tell whether we will have MongoDB support as a built-in one, but let me share a few quick comments:

• Follow the directory structure discussed at SQLAlchemy Session Store for Production Databases #1328
• Add some tests with mock objects that are helpful for long-term maintenance