fix(gitlab): discard label removal events on merge requests

[zakisk]

📝 Description of the Change

Label removal events on GitLab merge requests were incorrectly triggering pipeline runs. The hasOnlyLabelsChanged check used an OR condition that matched both additions and removals. Changed to compare current vs previous label count so only label additions are processed.

🔗 Linked GitHub Issue

Fixes #

🧪 Testing Strategy

• Unit tests
• Integration tests
• End-to-end tests
• Manual testing
• Not Applicable

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[gemini-code-assist]

Code Review

This pull request modifies the GitLab provider's event detection logic to determine if only labels have changed by comparing the length of current and previous labels. It also introduces a helper function and several test cases to verify different label change scenarios. The review feedback points out a critical bug in this implementation: comparing only the slice lengths fails to detect label changes when a label is added and another is removed simultaneously (as the total count remains unchanged). A code suggestion is provided to correctly identify when at least one new label has been added, along with a recommendation to add a corresponding test case for this scenario.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.20%. Comparing base (ffc5092) to head (9951a1d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2785      +/-   ##
==========================================
+ Coverage   60.19%   60.20%   +0.01%     
==========================================
  Files         211      211              
  Lines       21274    21280       +6     
==========================================
+ Hits        12806    12812       +6     
  Misses       7671     7671              
  Partials      797      797              

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[zakisk]

why lint rules are forced in this PR 🤔
https://github.com/tektoncd/pipelines-as-code/pull/2785/checks?check_run_id=81605577700

[zakisk]

/retest

[chmouel]

Code review: found 1 issue.