Add map_index_template support for mapped task group (#40799)

[anishgirianish]

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Summary

This adds map_index_template support to @task_group, so all child tasks in a mapped task group show meaningful labels instead of numeric indexes.

  @task_group(map_index_template="{{ filename }}")
  def file_transforms(filename):
      extracted = extract(filename)
      load(extracted)

Testing

• Unit tests across all layers (decorator, serialization, API, task runner)
• Manual E2E in breeze with three scenarios: basic chained tasks, task-level precedence, and partial() + expand()

closes #40799.

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[Dev-iL]

Looks good overall! Could benefit from some (docstring-level) clarifications.

[anishgirianish]

Thank you so much for the thorough review, @Dev-iL. I've addressed all the feedback, applied your match/case refactor, simplified the schema, added the iteration order comment, and added debug logging. Would really appreciate a re-review when you get a chance!

[uranusjr]

How is this different from setting the template on the tasks instead?