[ci]: move stats & CI status comment to workflow (#93220)
- Add a workflow_run reporter that posts PR test/stats comments using
`GITHUB_TOKEN`.
- Upload PR metadata artifacts from `build-and-test` and `Generate
Stats` so the reporter can safely resolve the target PR
- Move stats/test comment posting out of the external webhook/PAT path
while preserving the existing comment format
This means the test comment won't incrementally update as tests fail,
it'll just post once at the end, but that's a reasonable tradeoff to not
having to worry about a webhook.
