[.github] ensure all checks can handle multiple PRs opened from the same head_sha

### Specifics
* getIssueNumber() should search for all PRs (not just state:open), then choose the newest open PR, or the newest closed PR (if none open)
* Example failure on closed PR: https://github.com/Azure/azure-rest-api-specs/actions/runs/14063240357/job/39379132450#step:3:30

### Example
https://api.github.com/search/issues?q=f7c750c4751c75968036c37febf544179fbf68bc+type:pr+repo:azure/azure-rest-api-specs

https://github.com/Azure/azure-rest-api-specs/pull/33118/commits

https://github.com/Azure/azure-rest-api-specs/pull/33093/commits

https://github.com/Azure/azure-rest-api-specs/pull/32973/commits

Failed check: https://github.com/Azure/azure-rest-api-specs/actions/runs/13648977267/job/38153094326

Check failed with num_wf=0. If fallback to search API, might return multiple PRs. Then, how to process?