Fix short-circuit inside a mapped task group not skipping downstream

[kaxil]

A short-circuit task inside a mapped task group stopped skipping the task after it in 3.2.0.

Each map index of a task in a mapped task group runs as its own task instance, so SkipMixin writes the skip decision to XCom under that map index. #62287 changed NotPreviouslySkippedDep to read that XCom at map_index=-1 unless the parent is a MappedOperator, to fix an unmapped LatestOnlyOperator upstream of a mapped task. A task inside a mapped task group is a plain operator whose is_mapped is False, so the dep read -1, found no decision, and let the downstream task run.

The dep now checks get_needs_expansion(), which is true for a mapped operator and for any task inside a mapped task group: the tasks that write XCom per map index. An unmapped parent still reads -1, and the regression test from #62287 passes unchanged.

A gate that returns False for odd map indexes, gate(v) >> printstr(v) inside a task group expanded over [0, 1, 2, 3], run by the scheduler before and after the change:

related: #55225

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