Changes

fileserver: migrate tikv/copr-test artifacts to OCI registry (#4275)

`tikv/copr-test` pipeline was fetching `tikv-server` and `pd-server`
binaries from `fileserver.pingcap.net` via the legacy
`component.fetchAndExtractArtifact` helper. This migrates to OCI
artifacts on `hub-zot.pingcap.net/mirrors/hub`, consistent with the
pattern already used in
`tikv/pd/latest/pull_integration_copr_test.groovy`.

### Changes

- **`pod-pull_integration_test.yaml`** — add `utils` container
(`ghcr.io/pingcap-qe/cd/utils/release`) providing `oras`/`yq` for OCI
pulls
- **`pull_integration_test.groovy`**
- Replace `FILE_SERVER_URL` env var with `OCI_ARTIFACT_HOST =
'hub-zot.pingcap.net/mirrors/hub'`
- Compute OCI tags from PR metadata via
`component.computeArtifactOciTagFromPR()`
- Replace `component.fetchAndExtractArtifact(FILE_SERVER_URL, ...)` with
`download_pingcap_oci_artifact.sh` running in the `utils` container

```groovy
final OCI_TAG_TIKV = component.computeArtifactOciTagFromPR('tikv', REFS.base_ref, REFS.pulls[0].title, 'master')
final OCI_TAG_PD   = component.computeArtifactOciTagFromPR('pd',   REFS.base_ref, REFS.pulls[0].title, 'master')

// in Prepare stage, utils container:
${WORKSPACE}/scripts/artifacts/download_pingcap_oci_artifact.sh \
    --tikv=${OCI_TAG_TIKV} \
    --pd=${OCI_TAG_PD}
```

<!-- 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
tikv/copr-test</issue_title>
> <issue_description>Replace fileserver.pingcap.net usage for
tikv/copr-test with OCI artifacts in a standard registry.
> 
> Acceptance criteria:
> - No fileserver.pingcap.net references in jobs/pipelines for
tikv/copr-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#4214

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
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Co-authored-by: wuhuizuo <wuhuizuo@126.com>
 (commit: 9a6fb23)