Implement get_log_template on DagRunPydantic (apache#38303)

Also one drive-by fix removing unnecessary provide_session.

airflow/serialization/pydantic/dag_run.py

@@ -75,11 +75,10 @@ def get_task_instances(
         """
         raise NotImplementedError()
 
-    @provide_session
     def get_task_instance(
         self,
         task_id: str,
-        session: Session = NEW_SESSION,
+        session: Session,
         *,
         map_index: int = -1,
     ) -> TI | TaskInstancePydantic | None:
@@ -99,6 +98,11 @@ def get_task_instance(
             map_index=map_index,
         )
 
+    def get_log_template(self, session: Session):
+        from airflow.models.dagrun import DagRun
+
+        return DagRun._get_log_template(log_template_id=self.log_template_id)
+
 
 if is_pydantic_2_installed():
     DagRunPydantic.model_rebuild()