Fix branch not skipping mapped tasks with custom XCom backend

[darshil929]

When a custom XCom backend stores values externally (e.g. in S3), a branch operator stops skipping some of its downstream tasks on Airflow 3 — specifically dynamically-mapped tasks and tasks that are cleared and re-run.

Why: the branch records its skip decision as an XCom, and that XCom is pushed through your custom backend, which only persists a pointer to the real value. The scheduler reads that XCom as plain JSON to decide what to skip, can't interpret the pointer, and lets the task run instead of skipping it. It's the same root cause as the original 2.10.5 NoCredentialsError — on Airflow 3 the scheduler no longer calls the backend, so the crash turned into a silent skip failure.

Fix: keep the skip decision out of the custom backend. It's now stored in the same plain form Airflow uses by default, so the scheduler can always read it. Nothing changes when no custom backend is configured, and there's no Execution API change.

Added a regression test that runs a real branch operator with an externalizing backend configured — it fails before this change and passes after.

Separate from #52219, which was a different skip bug already fixed in #53455.

closes: #50491

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Was generative AI tooling used to co-author this PR?

• Yes — claude sonnet 4.6

[boring-cyborg]

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[eladkal]

Please provide evidance of before and after images of a running dag with the setup of custom backend.