Redo #59691: placeholder upstream map-index resolution breaks per-index expansion in mapped task groups

[potiuk]

#59691 ("Fix upstream map index resolution after placeholder expansion") was reverted in #68418 because it regressed per-index trigger-rule evaluation inside mapped task groups.

What #59691 did: added _should_use_post_expansion_placeholder in taskinstance.py, which rewrites a downstream placeholder's upstream resolution from map_index=-1 to 0 once the upstream has expanded.

Why it broke main: the same helper feeds both XCom/arg resolution (the case #59691 targeted) and trigger-rule dependency evaluation. For a task in a mapped task group whose upstream is in the same group (e.g. divide(i) >> report_success(i)), rewriting the summary instance (-1) to upstream index 0 makes it inherit that single instance's state — so when divide(0) fails, the downstream summary is marked UPSTREAM_FAILED and never expands. This regressed the #50210 fix; test_one_failed_trigger_rule_in_mapped_task_group_is_per_index (and the whole Core...Serialization job) went red on main.

Acceptance criteria for a redo:

• test_downstream_placeholder_handles_upstream_post_expansion (the Fix upstream map index resolution after placeholder expansion #59691 scenario) passes, and
• test_one_failed_trigger_rule_in_mapped_task_group_is_per_index + test_one_failed_trigger_rule_runs_on_indirect_failure_in_mapped_task_group (the Unexpected behavior of dynamically generated task with TriggerRule ONE_FAILED #50210 scenarios) pass.
• The placeholder→0 rewrite must not affect trigger-rule evaluation of not-yet-expanded summary instances (consider distinguishing the XCom-resolution caller, or handling the summary instance in trigger_rule_dep.py).

cc @SameerMesiah97

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Drafted-by: Claude Code (Opus 4.8); reviewed by @potiuk before posting