refactor: remove unnecessary 'client' fixture and update test durations (#4283)

* Remove unnecessary use of 'client' fixture in test_superuser

* Remove unnecessary use of 'usefixtures' decorator in test files

* Update test fixtures to improve test duration tracking

* Remove unnecessary 'client' fixture from test cases

src/backend/tests/unit/test_database.py

@@ -632,7 +632,6 @@ async def test_load_flows(client: TestClient):
     assert response.json()["folder_id"] is not None
 
 
-@pytest.mark.usefixtures("client")
 def test_sqlite_pragmas():
     db_service = get_db_service()