chore(ci): fix closed PR cache cleanup

[ytallo]

Summary

• force the cache-list request to use the GitHub Actions cache endpoint with GET
• propagate list-request failures before attempting cache deletion
• handle closed pull requests that have no matching caches
• add workflow contract coverage for the request method and failure behavior

Root cause

Passing -f and -F fields to gh api without an explicit method switched the request to POST. The cache-list endpoint rejected that request with 404. Because the request ran inside process substitution, the failure did not stop mapfile, and the error response was later treated as a cache ID.

Impact

Closed pull request caches can be enumerated and deleted again, reducing stale cache accumulation without attempting malformed deletion URLs.

Validation

• python3 -m pytest -q .github/scripts/tests/ - 188 passed, 3 subtests passed
• actionlint 1.7.12
• extracted cleanup shell validated with bash -n
• git diff --check
• live read-only GET against refs/pull/811/merge returned 8 matching caches

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[github-actions]

skill-check — worker

0 verified, 60 skipped (no docs/).

Layer	Result
structure	✓
vale	✓
ai	✓
render	✓

Four for four. Nicely done.