Changes

fix(flaky-tests): reopen issues using build start time (#4551)

### What

- Adjust flaky issue reopen logic for top N cases.

### How

- Reopen check now compares (same branch) the latest build start time
that observed the flaky case vs GitHub Issue `closed_at`.
- If build start > `closed_at`, reopen the issue and append a
reopen-evidence comment (includes the build link + timestamps).

### Notes

- Updated docs + unit tests in `tools/reporters/ci/flaky-tests`.
 (commit: 9841b0e)