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.

先在啟用 merge queue 和 most-recent-push approval 的情況下重現直到 PR B 的 stacked merge，然後追蹤 PR B 的 approval 失效後的 merge-queue 和 merge-dialog 流程。完成的標準是：該操作要麼合併兩個 PR，要麼在任何一個 merge 之前乾淨地停止，且不會讓對話框一直處於載入狀態。

由索引模型根據 Issue 內容生成。