Changes

refactor(pingcap-qe/tidb-test): migrate pingcap-qe/tidb-test artifacts to OCI registry (#4271)

All `fileserver.pingcap.net` references in jobs/pipelines for
`pingcap-qe/tidb-test` replaced with OCI artifact downloads via
`hub.pingcap.net` (configurable via `OCI_ARTIFACT_HOST`).

## Changes

### Latest branch — integration test pipelines (6 pipelines)
- Replaced `component.fetchAndExtractArtifact(FILE_SERVER_URL, ...)`
with `download_pingcap_oci_artifact.sh` in a `utils` container
- Added `OCI_TAG_PD` / `OCI_TAG_TIKV` constants via
`component.computeArtifactOciTagFromPR`
- Removed `FILE_SERVER_URL` env var; added `utils` container to each
`pod.yaml`

```groovy
final OCI_TAG_PD = component.computeArtifactOciTagFromPR('pd', REFS.base_ref, REFS.pulls[0].title, 'master')
final OCI_TAG_TIKV = component.computeArtifactOciTagFromPR('tikv', REFS.base_ref, REFS.pulls[0].title, 'master')
// ...
container("utils") {
    dir("bin") {
        retry(3) {
            sh "${WORKSPACE}/scripts/artifacts/download_pingcap_oci_artifact.sh --pd=${OCI_TAG_PD} --tikv=${OCI_TAG_TIKV}"
        }
    }
}
```

### Latest branch — tiproxy pipelines (5 pipelines)
- Replaced `./download_binary.sh` (internal fileserver caller) with
`download_pingcap_oci_artifact.sh` via `utils` container
- Added `utils` container to each `pod.yaml`

### Latest branch — simple cleanups
- Removed unused `FILE_SERVER_URL` env var from `ghpr_build` and
`ghpr_mysql_test`

### Release branches (6.0, 6.1, 6.2, 7.1, 8.5)
- Removed unused `FILE_SERVER_URL` from `ghpr_build` / `ghpr_mysql_test`
across all branches
- Full OCI migration for `ghpr_common_test`,
`ghpr_integration_{common,jdbc,mysql}_test` in release-7.1 and
release-8.5; `ghpr_integration_mysql_test` in release-6.1
- Added `utils` container to corresponding pod templates

### Script
- `scripts/PingCAP-QE/tidb-test/download_pingcap_artifact_v2.sh`:
rewrote to delegate to `download_pingcap_oci_artifact.sh` instead of
directly fetching from fileserver

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>fileserver: migrate artifacts for
pingcap-qe/tidb-test</issue_title>
> <issue_description>Replace fileserver.pingcap.net usage for
pingcap-qe/tidb-test with OCI artifacts in a standard registry.
> 
> Acceptance criteria:
> - No fileserver.pingcap.net references in jobs/pipelines for
pingcap-qe/tidb-test
> - Artifacts pulled via OCI registry with documented paths
> </issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes PingCAP-QE/ci#4216

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wuhuizuo <2574558+wuhuizuo@users.noreply.github.com>
Co-authored-by: wuhuizuo <wuhuizuo@126.com>
 (commit: c0534cb)