ci: prove bug regressions on destination

[kprokopenko]

What changed

Add a regression-proof workflow for bug-fix pull requests.

Behavior

• The job runs only when the PR has the bug label.
• It extracts changed *_test.go files and applies only that test patch to the destination revision.
• It runs the destination test suite and requires at least one concrete Go test event with Action == "fail".
• Build and package-level failures do not count as regression evidence.
• Source-branch success remains covered by the regular test checks.

The workflow listens to opened, synchronize, reopened, and labeled; removing a label does not start a run.

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[github-actions]

summary

Inferred base version: v3.144.0
Suggested version: v3.144.1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.25%. Comparing base (69cd174) to head (8236449).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2242      +/-   ##
==========================================
- Coverage   75.28%   75.25%   -0.03%     
==========================================
  Files         443      443              
  Lines       45935    45935              
==========================================
- Hits        34580    34567      -13     
- Misses       9400     9409       +9     
- Partials     1955     1959       +4     

Flag	Coverage Δ
experiment	75.15% <ø> (-0.07%)	⬇️
go-1.21.x	72.10% <ø> (-0.02%)	⬇️
go-1.26.x	75.23% <ø> (-0.05%)	⬇️
integration	56.73% <ø> (-0.06%)	⬇️
macOS	45.18% <ø> (ø)
ubuntu	75.24% <ø> (-0.04%)	⬇️
unit	45.18% <ø> (-0.02%)	⬇️
windows	45.18% <ø> (-0.01%)	⬇️
ydb-24.4	56.26% <ø> (-0.08%)	⬇️
ydb-edge	56.58% <ø> (-0.06%)	⬇️
ydb-latest	56.44% <ø> (-0.06%)	⬇️
ydb-nightly	75.15% <ø> (-0.07%)	⬇️

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