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.

[codecov]

Codecov Report

Attention: Patch coverage is 89.47368% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.47%. Comparing base (3994172) to head (211d06c).
Report is 2 commits behind head on release-8.5.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-8.5    #8831      +/-   ##
===============================================
- Coverage        75.59%   75.47%   -0.13%     
===============================================
  Files              461      462       +1     
  Lines            72296    72332      +36     
===============================================
- Hits             54655    54595      -60     
- Misses           14140    14223      +83     
- Partials          3501     3514      +13     

Flag	Coverage Δ
unittests	75.47% <89.47%> (-0.13%)	⬇️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JmPotato, lhy1024

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• OWNERS [JmPotato,lhy1024]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-06 03:07:37.443305194 +0000 UTC m=+1383445.062959710: ☑️ agreed by lhy1024.
• 2024-12-06 04:32:51.251135182 +0000 UTC m=+1388558.870789697: ☑️ agreed by JmPotato.