Stop cancelling in-progress main coverage runs

[glen-84]

Summary

• The Main Branch Coverage workflow used concurrency.cancel-in-progress: true, which killed an already-running coverage run whenever a newer commit landed on main, surfacing the cancelled run as a red status in the commit list.
• Drop cancel-in-progress (defaults to false), switching to queue mode: a running coverage job now finishes, and only superseded pending runs are cancelled. Serialization is preserved, so the latest commit's run still finishes last.
• Key the concurrency group per ref (main-coverage-${{ github.ref }}) so main and main-version-* branches serialize independently rather than cancelling each other.

Test plan

• Confirmed the workflow YAML parses and the concurrency block is well-formed.
• Verified against GitHub concurrency semantics: with cancel-in-progress unset, in-progress runs are never auto-cancelled and at most one pending run is retained per group, giving latest-wins ordering without killing active runs.

[Copilot]

Pull request overview

Adjusts GitHub Actions concurrency behavior for the “Main Branch Coverage” workflow so coverage runs on main aren’t marked red due to automatic cancellation when new commits land, while preserving serialized execution.

Changes:

• Removes concurrency.cancel-in-progress: true so in-progress coverage runs are no longer auto-cancelled by newer commits.
• Keys the concurrency group by ref (main-coverage-${{ github.ref }}) to serialize main and main-version-* independently.

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