ci: resolve PR number from head SHA in benchmark report

[zeitlinger]

Summary

The PR Benchmark Report workflow failed to comment on #2252 after a successful benchmark run (run 28865892835):

gh: Not Found (HTTP 404)
parse "https://api.github.com/repos/prometheus/client_java/issues/comments/{\r": net/url: invalid control character in URL

Root cause: workflow_run.pull_requests is empty when the triggering run's head branch is not on the default branch (or the PR is from a fork). With PR_NUMBER empty, the subsequent repos/${REPO}/issues//comments API call 404s and the paginator emits a malformed URL, so no comment gets posted.

Changes

• Fall back to repos/{owner}/{repo}/commits/{sha}/pulls to resolve the PR from the head SHA when workflow_run.pull_requests is empty.
• Fail fast with an explicit error if the PR still can't be resolved, so future regressions don't cascade into cryptic URL-parse errors.

Test plan

• Re-run the report workflow against run 28861406560 (or a fresh push to perf: optimize classic-only histogram observe path #2252) and confirm a benchmark comment lands on the PR.