CI: Coverage merge artifact download fails after retries

## Summary

The `Matrixone Utils CI / Coverage` check failed for PR #28402 while downloading coverage producer artifacts. The failure is in the GitHub Actions artifact transfer/extraction step, before coverage profiles are selected or merged; it is independent of the PR's MatrixOne source changes.

## Source PR and exact head

- PR: https://github.com/matrixorigin/matrixone/pull/28402
- Issue: https://github.com/matrixorigin/matrixone/issues/28347
- Exact head: `2a4edf2a1321aa175a57f80bc284d301df7e7e20`
- Branch: `m-28347`

## Failed check

- Check: `Matrixone Utils CI / Coverage`
- Run (attempt 1): https://github.com/matrixorigin/matrixone/actions/runs/34210960858
- Failed job: https://github.com/matrixorigin/matrixone/actions/runs/34210960858/job/102024675630
- Job ID: `102024675630`
- Failed step: `Download test coverage artifacts`

## Evidence

The failed job found three expected artifacts for run generation `34210960858-1`:

```text
bvt-coverage-pessimistic-generation-34210960858-1-attempt-1
bvt-coverage-compose-generation-34210960858-1-attempt-1
ut-coverage-generation-34210960858-1-attempt-1
```

The job log shows two downloads completing with SHA256 digests, while the remaining download did not produce a digest and the action ended with:

```text
##[error]Unable to download artifact(s): Unable to download and extract artifact: Artifact download failed after 5 retries.
```

The following merge and gate steps were skipped. There was no Go compilation error, failed test assertion, panic, static-analysis error, or coverage-threshold result. The upstream regular UT coverage producer and both BVT coverage producers completed successfully and published their artifacts; the three artifact records are present and not expired. This is direct hosted-runner/GitHub Actions artifact-service evidence, not a product-code failure.

The PR changes mo-service conntrack handling, compose container capabilities, sysview metadata, and distributed expected results. It does not change the coverage merge workflow or artifact download action. The changed-code and BVT producer paths in the same exact-head run completed successfully.

## Reproduction

1. Run MatrixOne ALL CI for the exact head above.
2. Open the Coverage job and inspect `Download test coverage artifacts`, which invokes `actions/download-artifact` for `*coverage*generation-34210960858-1-*` from run `34210960858`.
3. Observe the artifact list, the successful downloads, and the remaining download failing after five retries with the signature above; profile merge does not start.

This issue records the infrastructure failure for the exact PR run. The failed jobs from this run will be rerun once after issue creation.

Start with the Matrixone Utils CI / Coverage job from run 34210960858, especially the `Download test coverage artifacts` step that invokes `actions/download-artifact` and its retry logs. Reproduce the exact-head run and verify the three named coverage artifacts; done means artifact download and extraction complete so the coverage merge and gate steps run.

Written by the indexing model from the issue text.