fix: use Check Runs API for Codecov status in CI Gate

[fiddur]

Summary

• Fixes the Codecov check in CI Gate from fix: CI Gate checks Codecov status when backend tests ran #283 — it was querying the wrong GitHub API
• Codecov posts results as Check Runs (Checks API), not commit statuses (Status API)
• The previous implementation always got empty results and timed out for 5 minutes per check before passing with a warning

Change

Switches from repos/{repo}/commits/{sha}/status (statuses) to repos/{repo}/commits/{sha}/check-runs (check runs), and checks the conclusion field (success/neutral = pass, anything else = fail).

CI Gate should now complete in seconds after Codecov reports, instead of always waiting 10 minutes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.43%. Comparing base (dd07b1a) to head (bcd764f).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #284   +/-   ##
========================================
  Coverage    31.43%   31.43%           
========================================
  Files          181      181           
  Lines        22986    22986           
  Branches      1856     1856           
========================================
  Hits          7225     7225           
  Misses       15727    15727           
  Partials        34       34           

Flag	Coverage Δ
backend	58.08% <ø> (ø)

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