fileserver: migrate pingcap/tidb release-8.5 artifact downloads to OCI (#4293)
## Summary - split out `pipelines/pingcap/tidb/release-8.5/**` changes from #4274 into a standalone PR - migrate `release-8.5` pipelines away from `fileserver.pingcap.net` to OCI artifact pulls - keep implementation pattern aligned with migrated `latest` jobs (`OCI_TAG_*` + `download_pingcap_oci_artifact.sh` via `utils` container)
## Scope - 30 files under `pipelines/pingcap/tidb/release-8.5/`
## Validation - Jenkinsfile validation succeeded for all changed Groovy files via `.ci/verify-jenkins-pipeline-file.sh`