fix: remove unused db variable in exception context manager test

Author: Rafi-Mohammad-69

src/tests/backend/common/database/test_database_base.py

@@ -495,7 +495,7 @@ async def get_team_agent(self, team_id, agent_name): return None
         database = MockDatabase()
 
         async def run_database_context():
-            async with database as db:
+            async with database:
                 assert database.initialized is True
                 # Raise an exception to test cleanup
                 raise ValueError("Test exception")