Fix branch operators inside mapped task groups not skipping siblings

[sohailiqrar]

When a @task.branch lives inside a mapped @task_group it is not itself expanded (parent.is_mapped is False), but its TI runs at the group's map_index and SkipMixin writes the skip XCom there — not at -1. NotPreviouslySkippedDep was querying map_index=-1 for every unmapped parent, finding no XCom, and letting all siblings proceed regardless of what the branch decided.

Detect whether the parent ran in the same mapped context by checking for a finished parent TI at the child's map_index. If one exists, use that index for the XCom lookup; fall back to -1 only for truly unmapped parents (e.g. LatestOnlyOperator) that always write at -1.

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[kaxil]

Closing this. #73959 fixed this on main: NotPreviouslySkippedDep now reads xcom_map_index = ti.map_index if parent.get_needs_expansion() else -1, which covers parents inside a mapped task group. The new test also sets map_index = 1 by hand on unmapped tasks rather than building a mapped task group, so it doesn't exercise the case.