Changes

fix(tidb-test): use component.fetchAndExtractArtifact for artifact download (#3959)

Replace direct shell script call with component.fetchAndExtractArtifact
to automatically handle branch fallback for hotfix branches.

Changes:
- Replace download_pingcap_artifact.sh with
component.fetchAndExtractArtifact
- Automatically fallback from hotfix branch (e.g.,
release-8.5-20251125-v8.5.4) to release branch (e.g., release-8.5) when
artifact doesn't exist
- Fix 404 errors when downloading tikv/pd artifacts for hotfix branches
 (commit: b0b6dc7)