fix(tmdb): harden aiohttp session lifecycle

- Add async context manager to NFOService wrapping TMDBClient + ImageDownloader
- Add TMDBClient.__del__ warning when session leaks
- Log exc_info on session recreation for traceback visibility
- Document async-with usage in docs/DEVELOPMENT.md and docs/TESTING.md
- Add unit tests covering leak detection, context-manager cleanup, and connector-closed warning

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/TESTING.md

@@ -108,6 +108,44 @@ class MockQueueRepository:
 - `save_item` uses `item.id` as key (must be set before calling)
 - Suitable for unit tests only (no persistence)
 
+### Mocking aiohttp Sessions
+
+When testing code that uses `aiohttp.ClientSession`:
+
+```python
+from unittest.mock import AsyncMock, MagicMock, patch
+from aiohttp import ClientSession
+
+# Mock aiohttp session for testing
+class MockAiohttpSession:
+    def __init__(self):
+        self.closed = False
+    
+    async def close(self):
+        self.closed = True
+    
+    def get(self, url, **kwargs):
+        mock_response = AsyncMock()
+        mock_response.status = 200
+        mock_response.json = AsyncMock(return_value={"data": "test"})
+        mock_response.__aenter__ = AsyncMock(return_value=mock_response)
+        mock_response.__aexit__ = AsyncMock(return_value=None)
+        return mock_response
+
+# Use in fixture
+@pytest.fixture
+async def mock_tmdb_session():
+    session = MockAiohttpSession()
+    yield session
+    # Cleanup verification
+    assert session.closed, "Session was not closed"
+```
+
+**Key points:**
+- Always verify `session.closed` is `True` after context manager exits
+- Mock `__aenter__` and `__aexit__` for response context managers
+- Set `closed = False` on mock session for unclosed warning tests
+
 7. Coverage Requirements
 8. CI/CD Integration
 9. Writing Good Tests