syncer: exit watch leader immediately (#8824)

[ti-chi-bot]

This is an automated cherry-pick of #8824

What problem does this PR solve?

Issue Number: Close #8823

What is changed and how does it work?

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nolouch

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tikv/pd/blob/release-7.5/OWNERS)~~ [nolouch] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-11-21 09:22:02.397284107 +0000 UTC m=+109910.016938607: :ballot_box_with_check: agreed by nolouch.