Fix MyPy type errors in test_dags.py (apache#56735) (apache#56770)

* Fix MyPy type errors in test_dags.py

* Replace datetime with pendulum for UTC timezone handling in test_dags.py

Author: choo121600

airflow-core/tests/unit/api_fastapi/core_api/routes/ui/test_dags.py

@@ -16,7 +16,6 @@
 # under the License.
 from __future__ import annotations
 
-from datetime import datetime, timezone
 from typing import TYPE_CHECKING
 from unittest import mock
 
@@ -56,7 +55,7 @@ def setup_dag_runs(self, session=None) -> None:
         for dag_id in [DAG1_ID, DAG2_ID, DAG3_ID, DAG4_ID, DAG5_ID]:
             dag_runs_count = 5 if dag_id in [DAG1_ID, DAG2_ID] else 2
             for i in range(dag_runs_count):
-                start_date = datetime(2021 + i, 1, 1, 0, 0, 0, tzinfo=timezone.utc)
+                start_date = pendulum.datetime(2021 + i, 1, 1, 0, 0, 0, tz="UTC")
                 dag_run = DagRun(
                     dag_id=dag_id,
                     run_id=f"run_id_{i + 1}",
@@ -67,7 +66,8 @@ def setup_dag_runs(self, session=None) -> None:
                     state=(DagRunState.FAILED if i % 2 == 0 else DagRunState.SUCCESS),
                     triggered_by=DagRunTriggeredByType.TEST,
                 )
-                dag_run.end_date = dag_run.start_date + pendulum.duration(hours=1)
+                if dag_run.start_date is not None:
+                    dag_run.end_date = dag_run.start_date.add(hours=1)
                 session.add(dag_run)
         session.commit()