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.

Written by the indexing model from the issue text.