Stack merge hangs after partial merge invalidates latest-push approval

## Reproduction

1. Create `main <- PR A <- PR B`.
2. Enable merge queue and "Require approval of the most recent reviewable push."
3. Approve both PRs after their latest pushes.
4. Merge the stack from PR B.

## Expected

Both PRs merge, or the operation stops before either merges.

## Actual

PR A merges. GitHub rebases PR B, invalidating its approval. PR B is not queued, and the merge dialog spins indefinitely.

Start by reproducing the stacked merge through PR B with merge queue and most-recent-push approval enabled, then trace the merge-queue and merge-dialog flow after PR B's approval is invalidated. Done means the operation either merges both PRs or stops cleanly before either merge, without leaving the dialog spinning.

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