fix: replace deprecated .dict() with .model_dump() and fix db.add() AsyncMock warnings

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/ac39d3f7-d4c3-4417-8c33-8e6340a27281

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/tests/unit/test_tasks.py

@@ -77,6 +77,8 @@ def _mock_session_maker():
     is usable as ``async with session_maker_mock() as db:``.
     """
     session = AsyncMock()
+    # db.add() is synchronous in SQLAlchemy; prevent "coroutine never awaited" warnings
+    session.add = MagicMock()
 
     # Make the session usable as an async context manager
     ctx = AsyncMock()