Fix task_instance_mutation_hook when importing airflow.models.dagrun (

#15851)

If a dag imported `airflow.models.dagrun` it would cause task_instance_mutation_hook from the site local settings to not be picked up.

airflow/models/dagrun.py

@@ -41,7 +41,6 @@
 from airflow.exceptions import AirflowException, TaskNotFound
 from airflow.models.base import ID_LEN, Base
 from airflow.models.taskinstance import TaskInstance as TI
-from airflow.settings import task_instance_mutation_hook
 from airflow.stats import Stats
 from airflow.ti_deps.dep_context import DepContext
 from airflow.ti_deps.dependencies_states import SCHEDULEABLE_STATES
@@ -633,6 +632,8 @@ def verify_integrity(self, session: Session = None):
         :param session: Sqlalchemy ORM Session
         :type session: Session
         """
+        from airflow.settings import task_instance_mutation_hook
+
         dag = self.get_dag()
         tis = self.get_task_instances(session=session)
 

tests/models/test_dagrun.py

@@ -611,7 +611,7 @@ def test_already_added_task_instances_can_be_ignored(self):
             assert State.NONE == first_ti.state
 
     @parameterized.expand([(state,) for state in State.task_states])
-    @mock.patch('airflow.models.dagrun.task_instance_mutation_hook')
+    @mock.patch('airflow.settings.task_instance_mutation_hook')
     def test_task_instance_mutation_hook(self, state, mock_hook):
         def mutate_task_instance(task_instance):
             if task_instance.queue == 'queue1':