fix(coalescer): treat CoalescingRefused as the documented safe no-op in main() - #1822

Summary

• current-head-run-coalescer.yml's own design comment states that CoalescingRefused (a queued run's remembered head no longer matching the PR's live head) is "a safe no-op." coalesce()'s top-level live-PR-state check raises exactly that exception before any per-candidate cancellation loop even starts — but main() only ever called coalesce() directly with no handling for that exception, so it propagated uncaught and crashed the required coalesce check with exit code 1, instead of the intended graceful no-op.
• Live evidence of the real breakage: ContextualWisdomLab/.github#1503, run 33766056421, job coalesce — a queued run drained from the org-wide Actions capacity backlog against an already-superseded head failed with CoalescingRefused: pull request head moved before duplicate classification.
• Fix: main() now catches CoalescingRefused specifically and exits 0 with an informational message, matching the treatment the per-candidate loop inside coalesce() already gives the same exception type. Any other exception (malformed repository/PR identity, an unavailable GitHub API) still fails closed — this is a narrow, intentional carve-out, not a general fail-open.

Changes

• scripts/ci/current_head_run_coalescer.py: wrap the coalesce() call in main() in a try/except CoalescingRefused.
• tests/test_current_head_run_coalescer.py: new regression test test_main_treats_coalescing_refused_as_a_safe_no_op, confirmed failing (uncaught exception propagating) against the pre-fix main() before the fix landed.
• CHANGELOG.md: ## [Unreleased] entry describing the fix and the live incident it responds to.

Verification

coverage run -m pytest tests && coverage report --show-missing
# 2762 passed, 1 skipped, 21 subtests passed
# (the 1 skip is 2 pre-existing files incompatible with local Python 3.11 vs. this repo's pinned 3.12+/3.14 CI target — unrelated to this change)
# TOTAL coverage (scripts/ci/*): 100%

interrogate scripts/ci/current_head_run_coalescer.py
# RESULT: PASSED (minimum: 100.0%, actual: 100.0%)

git diff --check
# clean

RED confirmed: the new test fails against the pre-fix main() with the CoalescingRefused exception propagating uncaught. GREEN confirmed after the fix.

Notes for reviewers

• This is a .github-central-repo-only change (no workflow YAML edits) to a script the trusted current-head-run-coalescer.yml pull_request_target workflow already calls. No bypass-merge request — normal review is fine.
• Scope is intentionally narrow: only the specific, already-documented-as-safe CoalescingRefused signal is caught. Malformed-input RuntimeErrors from coalesce()'s own validation still propagate and fail closed, unchanged.

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