Fix SQLA Mapped datetime imports in models

[amoghrajesh]

Along similar vein as #56253

Latest issue:

  NameError: Could not de-stringify annotation 'datetime | None'
  
  The above exception was the direct cause of the following exception:
  
  Traceback (most recent call last):
    File "/usr/python/bin/airflow", line 4, in <module>
      from airflow.__main__ import main
    File "/opt/airflow/airflow-core/src/airflow/__main__.py", line 37, in <module>
      from airflow.cli import cli_parser
    File "/opt/airflow/airflow-core/src/airflow/cli/cli_parser.py", line 40, in <module>
      from airflow.cli.cli_config import (
      ...<5 lines>...
      )
    File "/opt/airflow/airflow-core/src/airflow/cli/cli_config.py", line 36, in <module>
      from airflow.utils.cli import ColorMode
    File "/opt/airflow/airflow-core/src/airflow/utils/cli.py", line 39, in <module>
      from airflow.dag_processing.dagbag import DagBag
    File "/opt/airflow/airflow-core/src/airflow/dag_processing/dagbag.py", line 52, in <module>
      from airflow.serialization.serialized_objects import LazyDeserializedDAG
    File "/opt/airflow/airflow-core/src/airflow/serialization/serialized_objects.py", line 69, in <module>
      from airflow.models.dag import DagModel
    File "/opt/airflow/airflow-core/src/airflow/models/dag.py", line 55, in <module>
      from airflow.models.dagrun import DagRun
    File "/opt/airflow/airflow-core/src/airflow/models/dagrun.py", line 64, in <module>
      from airflow.models.backfill import Backfill
    File "/opt/airflow/airflow-core/src/airflow/models/backfill.py", line 114, in <module>
      class Backfill(Base):
      ...<39 lines>...
              return f"Backfill({self.dag_id=}, {self.from_date=}, {self.to_date=})"
    File "/usr/python/lib/python3.13/site-packages/sqlalchemy/orm/decl_api.py", line 198, in __init__
      _as_declarative(reg, cls, dict_)
      ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
    File "/usr/python/lib/python3.13/site-packages/sqlalchemy/orm/decl_base.py", line 245, in _as_declarative
      return _MapperConfig.setup_mapping(registry, cls, dict_, None, {})
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/usr/python/lib/python3.13/site-packages/sqlalchemy/orm/decl_base.py", line 326, in setup_mapping
      return _ClassScanMapperConfig(
          registry, cls_, dict_, table, mapper_kw
      )
    File "/usr/python/lib/python3.13/site-packages/sqlalchemy/orm/decl_base.py", line 573, in __init__
      self._extract_mappable_attributes()
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
    File "/usr/python/lib/python3.13/site-packages/sqlalchemy/orm/decl_base.py", line 1579, in _extract_mappable_attributes
      raise orm_exc.MappedAnnotationError(
      ...<4 lines>...
      ) from ne
  sqlalchemy.orm.exc.MappedAnnotationError: Could not resolve all types within mapped annotation: "Mapped[datetime | None]".  Ensure all types are written correctly and are imported within the module in use.
  
  Error: check_environment returned 1. Exiting.
  

Example: https://github.com/apache/airflow/actions/runs/18135253335/job/51613587395

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.