sessions: gate CI rerun button on valid run ID and add tests (#306362)

sessions: gate rerun button on valid run ID and add tests

Only show the Rerun Check action when the check's detailsUrl contains
a parseable GitHub Actions workflow run ID. Export parseWorkflowRunId
for reuse.

Add unit tests for rerunFailedJobs endpoint and parseWorkflowRunId.

Co-authored-by: Copilot <[email protected]>