fix(automerge): validate head commit before merge

[loveulvu]

Fixes #39214.

Auto-merge already validates that the queued head commit still matches the pull request head before checking mergeability, but the expected head commit was not passed to the final merge operation.

Pass expectedHeadCommitID to pull_service.Merge so the head commit is validated again when the merge is actually performed.

The "automerge" related tests are already very messy, and it's very difficult to test the "validation" behaviors without data-race. Since the fix is simple, this PR doesn't introduce new tests.

[chatgpt-codex-connector]

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[wxiaoguang]

If it's difficult to write a correct test, then don't write the test, since the fix is simple

[chatgpt-codex-connector]

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