Support mark success for both upstream task and downstream mapped tasks in dag.test()

[kevinhongzl]

Summary

Previously marking both an upstream task and its downstream mapped tasks as successful could still fail during expansion. Although the mapped task was marked successful, we still needed upstream XComs to determine how many task instances to create. When the upstream task was also marked successful and therefore did not run, no XCom was produced, so expansion failed. This PR updates dag.test() to handle this scenario.

closes: #61558

Updated behavior

The behavior is now:

upstream (marked success) -> no XCom  -> mapped (marked success) -> create 1 successful TI
upstream (executed)       -> XCom [n] -> mapped (marked success) -> create n successful TIs
upstream (marked success) -> no XCom  -> mapped (executed) -> fail as before
upstream (executed)       -> XCom [n] -> mapped (executed) -> create n executed TIs

In other words, when a mapped task is marked successful:

• if upstream mapping data exists, we use it to create the expected number of mapped task instances
• if upstream mapping data does not exist because the upstream task was also marked successful, we create a single successful mapped task

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[SameerMesiah97]

The fix looks plausible to me.

My only concern is around the map_index=0 fallback when upstream expansion metadata does not exist. It makessense pragmatically for dag.test(), but this touches mapped TI materialization internals. I’d defer to a maintainer for final approval.