fix(github): query check runs for /retest

[theakshaypant]

📝 Description of the Change

Implement GetCommitStatuses for the GitHub provider so /retest only re-runs failed pipelines when PipelineRuns have been pruned.

The stub returned nil, causing filterSuccessfulTemplates to skip all pipelines through. Now queries the Check Runs API in GitHub App mode and the Commit Statuses API in PAT/webhook mode, with deduplication and retry-with-backoff matching the write path.

🔗 Linked GitHub Issue

Partially fixes #2580

🧪 Testing Strategy

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

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[pipelines-as-code]

Paco Review 🔍

This PR implements the previously stubbed GetCommitStatuses method for the GitHub provider, fetching either GitHub Check Runs (App mode) or classic commit statuses (PAT/webhook mode), deduplicating entries by name+status, and paginating through results. It also adds a comprehensive table-driven unit test suite for the new method and a large new e2e test verifying that /retest only reruns previously-failed pipelines after PipelineRuns have been pruned from the cluster.

Review difficulty: 3/5 (Moderate) — The core change is a single, moderately complex method with two distinct API code paths plus a sizeable new e2e test, giving it real but bounded blast radius.

1 new inline comment(s) found.

_{Reviewed commit: a09e1a4}

[pipelines-as-code]

Paco inline comments -- see the Paco Review summary comment for the overview.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.66%. Comparing base (c42a902) to head (348e268).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2876      +/-   ##
==========================================
+ Coverage   68.45%   68.66%   +0.20%     
==========================================
  Files         197      197              
  Lines       16766    16855      +89     
==========================================
+ Hits        11477    11573      +96     
+ Misses       4417     4414       -3     
+ Partials      872      868       -4     

Flag	Coverage Δ
unit-tests	68.66% <100.00%> (+0.20%)	⬆️

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[chmouel]

I don't know what's going on with go-testing job timing out super long these days

[zakisk]

LGTM, please address the last one comment