fix: add missing `/sessions/{session_id}/objects/{file_id}` endpoint for LibreChat file freshness check (#71)

* feat(api): add endpoint to retrieve session object metadata

* Implement GET /sessions/{session_id}/objects/{file_id} to return file object metadata.
* Include lastModified timestamp for active sessions.
* Ensure compatibility with LibreChat's getSessionInfo.
* Add tests for the new endpoint to validate functionality and error handling.

* fix(review): address PR #71 review comments

- Rename 'id' param to 'resource_id' with alias='id' to avoid shadowing builtin
- Log warning on session retrieval failure instead of silent except pass
- Return 500 for unexpected errors instead of misleading 404
- Assert ACTIVE branch timestamp is close to now in test
- Add test case for inactive session with last_activity