Fix race condition in _GENAI_CLIENT initialization using async lock

[kirandharmar867-maker]

Issue: _GENAI_CLIENT singleton not thread-safe on cold start

Root Cause:

The GenAI client was being initialized lazily without any synchronization.
When multiple requests hit the server at the same time (especially during cold start), they could all see _GENAI_CLIENT as None and create multiple instances.

Changes:
Added an asyncio.Lock to make initialization thread-safe.
Used double-check locking to ensure the client is created only once.
Converted _get_genai_client() to async.
Updated all usages to await _get_genai_client().

Impact:
Prevents duplicate client creation
Fixes race condition during concurrent requests
Makes initialization safe and stable
No change in existing functionality

Steps to Test:
Run the backend server
Send multiple requests at the same time
Verify only one client is initialized and everything works normally

Files Modified:
backend/agents.py