Fix async context manager usage in BackgroundLoaderService

- Changed 'async for' to 'async with' for get_db_session()
- get_db_session() is @asynccontextmanager, requires async with not async for
- Created 5 comprehensive unit tests verifying the fix
- All tests pass, background loading now works correctly

tests/unit/test_dependency_exception_handling.py

@@ -4,11 +4,12 @@ Unit tests for dependency exception handling in FastAPI dependencies.
 This module tests that async generator dependencies properly handle exceptions
 thrown back into them, preventing the "generator didn't stop after athrow()" error.
 """
-import pytest
-from fastapi import FastAPI, HTTPException, Depends
-from httpx import AsyncClient, ASGITransport
 from typing import AsyncGenerator, Optional
 
+import pytest
+from fastapi import Depends, FastAPI, HTTPException
+from httpx import ASGITransport, AsyncClient
+
 
 @pytest.mark.asyncio
 async def test_get_optional_database_session_handles_http_exception():
@@ -18,7 +19,7 @@ async def test_get_optional_database_session_handles_http_exception():
     that occurred when an HTTPException was raised after yielding a database session.
     """
     from src.server.utils.dependencies import get_optional_database_session
-    
+
     # Create a test app
     app = FastAPI()
     
@@ -48,7 +49,7 @@ async def test_get_database_session_handles_http_exception():
     that occurred when an HTTPException was raised after yielding a database session.
     """
     from src.server.utils.dependencies import get_database_session
-    
+
     # Create a test app
     app = FastAPI()