Changes

fix(flaky-tests): resolve build started time for reopen (#4552)

### Background

Follow-up to #4551: align reopen eligibility with the latest design by
resolving real CI build started time from `build_url` when possible.

### What

- Reopen check compares latest flaky build `started_at` (best-effort
from build URL) vs Issue `closed_at`.
- When reopened, always adds an evidence comment (branch/build
URL/timestamps + source).

### How

- Jenkins: `GET <build_url>/api/json?tree=timestamp`
- Prow view (gs/gcs): `GET <build_url>/started.json` (via GCS)
- Fallback: latest flaky `report_time` (max report_time where flaky >
0).

### Tests

- Updated `GithubIssueManager.test.ts` to cover
Jenkins/Prow/fallback/no-fallback cases.
 (commit: 3ab2cfc)