Stack fails to merge partway through when a branch gets automatically deleted and then the PR is automatically closed

When trying to merge the stack containing the following [PR](https://github.com/gradientlabs-ai/wearegradient/pull/32235). It was removed from the merged queue after the branch from the earlier PR was automatically deleted and the PR was then automatically closed.

Start by reproducing the stack merge involving PR 32235, then trace how the merged queue handles an earlier branch being automatically deleted and its PR being closed. Done means the stack no longer fails partway in this scenario and the behavior is covered by an appropriate regression test.

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