Fix : Validate mapped task map_index bounds in task CLI

[Nandann018-ux]

What changed

Added validation to reject out-of-range --map-index for mapped tasks in airflow tasks commands.
Added a unit test for tasks render to ensure out-of-range indexes raise TaskInstanceNotFound.
Added a newsfragment.
Why
Passing a map index beyond the mapped task count currently creates a TaskInstance that can never exist, leading to confusing behavior for CLI users. Validating bounds up front provides a clear, consistent error and avoids creating invalid rows.

[boring-cyborg]

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