Scheduler deserialization fails for mapped operators with templated date strings in partial_kwargs (Airflow 3.1.x)

### Apache Airflow version 3.1.5 ### If "Other Airflow 3 version" selected, which one? _No response_ ### What happened? On Airflow 3.1.x, a DAG using dynamic task mapping over a custom operator that has templated `from_date`/`to_date` in `partial_kwargs` parses, but the scheduler fails to deserialize the serialized DAG. **This is a regression from v3.0.x** ### What you think should happen instead? The scheduler should successfully deserialize the DAG. Templated strings in operator kwargs should remain strings and not be coerced to timestamps during deserialization. Here's the full traceback. ``` Traceback (most recent call last): 2025-12-16T16:47:02.151920509Z File "/usr/local/lib/python3.12/site-packages/airflow/serialization/serialized_objects.py", line 2552, in deserialize_dag 2025-12-16T16:47:02.152207759Z return cls._deserialize_dag_internal(encoded_dag, client_defaults) 2025-12-16T16:47:02.152214134Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 2025-12-16T16:47:02.152215759Z File "/usr/local/lib/python3.12/site-packages/airflow/serialization/serialized_objects.py", line 2578, in _deserialize_dag_internal 2025-12-16T16:47:02.152479051Z deser = SerializedBaseOperator.deserialize_operator( 2025-12-16T16:47:02.152481926Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 2025-12-16T16:47:02.152482759Z File "/usr/local/lib/python3.12/site-packages/airflow/serialization/serialized_objects.py", line 1774, in deserialize_operator 2025-12-16T16:47:02.152641176Z cls.populate_operator(op, encoded_op, client_defaults) 2025-12-16T16:47:02.152643842Z File "/usr/local/lib/python3.12/site-packages/airflow/serialization/serialized_objects.py", line 1616, in populate_operator 2025-12-16T16:47:02.152823384Z v = cls._deserialize_partial_kwargs(v, client_defaults) 2025-12-16T16:47:02.152827592Z ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 2025-12-16T16:47:02.152828509Z File "/usr/local/lib/python3.12/site-pac…