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]

Does this mean the group's expand input can't depend on mapped upstream task outputs? If so, this should be documented as a limitation.

[anishgirianish]

Great question! From what I can tell, the upstream producing the expand input is unmapped; it returns a list that drives the mapping. If it were mapped, it would go through expand_kwargs instead, so it wouldn't be a SchedulerXComArg here. But I'd love to hear if you've seen a case where this breaks,

[Dev-iL]

I'm not that familiar with xcom intricacies - perhaps it's worth adding a test that ensures this scenario works correctly.

[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!