fix(ci): cancel tests when pull requests close

[seonghobae]

Outcome

• emit a pull_request.closed Tests workflow run in the existing per-PR concurrency group
• let GitHub cancel the obsolete in-progress test run before a runner is allocated
• skip backend and frontend jobs on the close event, preserving normal push/open/synchronize/reopen coverage

This is stacked on #631 so its exact queue baseline remains the parent authority. After #631 merges, retarget this PR to main and re-verify the exact head.

Root-cause evidence

Two Tests runs for already merged PRs #627 and #628 continued occupying the available runner slots while 54 newer runs remained queued. Manual cancellation released the stale work, but the existing central close workflow was itself queued behind those runs. The repository Tests workflow already had the correct per-PR concurrency group; adding the close event lets GitHub apply cancel-in-progress without waiting for a job runner.

Verification

• uv run --extra dev pytest -q tests/test_tests_workflow_contract.py tests/test_documentation_hygiene.py (6 passed)
• actionlint .github/workflows/tests.yml
• git diff --check

No runtime product or UI surface changed; Figma and screenshot review do not apply.

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[devin-ai-integration]

Devin Review found 1 potential issue.