fix(actions): verify coalescer cancellation completion

[seonghobae]

Summary

• poll the exact Actions run after an accepted current-head coalescer cancellation
• require terminal completed/cancelled evidence instead of treating HTTP 202 as completion
• fail closed after six bounded one-second polls
• prove a failed terminal-state check cannot append a cancelled ID or emit a success report
• preserve the existing ordinary /cancel endpoint and per-request timeout

Verification

• uv run pytest tests/test_current_head_run_coalescer.py tests/test_current_head_run_coalescer_review_regressions.py -q — 50 passed
• git diff --check
• exact head: d764298469609ec498de91380d256fc65e449252

This implements the cancellation-completion rule documented in #1885 without mixing runtime code into the documentation PR.

[coderabbitai]

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Review configuration:

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Team

Run ID: 098b6fa8-545e-4a6b-b986-ec8d615bc290

📥 Commits

Reviewing files that changed from the base of the PR and between 0eb41a3 and 90fe695.

📒 Files selected for processing (3)

• scripts/ci/current_head_run_coalescer.py
• tests/test_current_head_run_coalescer.py
• tests/test_current_head_run_coalescer_review_regressions.py

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