Fix lock map_index_template when running mapped task

[LiuQhahah]

Fixes a bug where the scheduler would crash when trying to expand a mapped task. The MappedOperator object in the scheduler was missing the map_index_template attribute, leading to an
AttributeError.

Problem

• When the scheduler prepares a mapped task for execution, it builds a template context for the task instance.
• This process involves accessing the map_index_template attribute on the task object.
• The MappedOperator object used by the scheduler was missing this attribute, causing the scheduler to fail with an AttributeError: 'MappedOperator' object has no attribute 'map_index_template'.

Solution

• Add the map_index_template property to the MappedOperator class located in airflow-core/src/airflow/models/mappedoperator.py.
• This property now correctly retrieves the value from the operator's partial_kwargs, ensuring the attribute is available to the scheduler at runtime.

Testing

• The fix was validated by ensuring the scheduler no longer crashes when running DAGs with mapped tasks in the local environment
• Manual inspection confirms the map_index_template attribute is now correctly accessed.

reproduce :

Related Issues

Closes #54627

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.