labeler: fix deleting non-existing label rule should return 404 (#10090)

[ti-chi-bot]

This is an automated cherry-pick of #10090

What problem does this PR solve?

Issue Number: Close #10089 ref #9854

What is changed and how does it work?

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be LGTMed and approved by the reviewers firstly.
2. For pull requests to TiDB-x branches, it must have no failed tests.
3. AFTER it has lgtm and approved labels, please wait for the cherry-pick merging approval from triage owners.

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[ti-chi-bot]

@lhy1024 This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bufferflies

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Details

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-12-21 14:17:52.800706115 +0000 UTC m=+2001017.614483697: ☑️ agreed by bufferflies.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.87%. Comparing base (029eb6e) to head (4104ee4).

Additional details and impacted files

@@               Coverage Diff               @@
##           release-8.5   #10098      +/-   ##
===============================================
+ Coverage        77.81%   77.87%   +0.05%     
===============================================
  Files              482      482              
  Lines            65343    65343              
===============================================
+ Hits             50846    50885      +39     
+ Misses           10743    10719      -24     
+ Partials          3754     3739      -15     

Flag	Coverage Δ
unittests	77.87% <100.00%> (+0.05%)	⬆️

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