Add type casting for mypy to understand expand_input types

[amoghrajesh]

Mypy issue on main:

airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py:311: error: Item "DictOfListsExpandInput" of
"SchedulerDictOfListsExpandInput | SchedulerListOfDictsExpandInput | DictOfListsExpandInput | ListOfDictsExpandInput" has no attribute "get_total_map_length"  [union-attr]
                    mapped_ti_count = upstream_mapped_group._expand_input.get_total_map_length(
                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py:311: error: Item "ListOfDictsExpandInput" of
"SchedulerDictOfListsExpandInput | SchedulerListOfDictsExpandInput | DictOfListsExpandInput | ListOfDictsExpandInput" has no attribute "get_total_map_length"  [union-attr]
                    mapped_ti_count = upstream_mapped_group._expand_input.get_total_map_length(

Adding a type cast to help mypy understand that upstream_mapped_group._expand_inputis a SchedulerExpandInput at runtime which has get_total_map_length defined.

Safe cast because at runtime the expand input will always be a SchedulerExpandInput, we just need to help mypy understand this.

Example failure: https://github.com/apache/airflow/actions/runs/16221450919/job/45803515859

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[gopidesupavan]

lets see i will rebase mypy bump on this hopefully 🤞