fix: use core.setFailed() instead of throw Error() in find-exercise-issue workflow

[Claude]

Changes

The find-exercise-issue workflow was throwing unhandled errors when no exercise issue was found, resulting in GitHub Actions displaying "Error: Unhandled error: Error: No exercise issue found."

Changed error handling in .github/workflows/find-exercise-issue.yml to use core.setFailed() instead of throw new Error():

// Before
if (!issue) {
  throw new Error('No exercise issue found');
}

// After
if (!issue) {
  core.setFailed('No exercise issue found');
  return;
}

This matches the pattern used elsewhere in the codebase (e.g., actions/file-exists/action.yml) and ensures proper error formatting in GitHub Actions.

Checklist

• I have added or updated appropriate labels to this PR
• I have tested my changes
• I have updated the documentation if needed

[Copilot]

Pull request overview

Updates the find-exercise-issue workflow’s error handling to fail the GitHub Actions step cleanly (without an “Unhandled error”) when no matching exercise issue is found.

Changes:

• Replaced throw new Error(...) with core.setFailed(...) for missing-issue handling
• Added an early return after setting the failure state to stop further script execution

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