CI: coverage artifact listing times out in GitHub Actions

[yichao-liang]

The required coverage aggregation job can fail before reading any coverage data because GitHub's artifact service times out while listing artifacts.

Observed on run 34343743328, coverage job 102443736030 for PR #173 at commit 1095ad4eacd2731f2033957d1421618d0eccae9a.
All eight unit-test shards and the other five required checks passed.
Python setup and pip install coverage succeeded.
The actions/download-artifact@v4 step made five attempts to call ArtifactService/ListArtifacts, and each timed out.
It then failed with Unable to download artifact(s): Failed to ListArtifacts.

The run-artifacts REST endpoint subsequently returned all eight expected coverage-group-1 through coverage-group-8 artifacts, none expired.
Recovery is limited to rerunning the failed coverage aggregation job against those existing artifacts; the passing tests are not rerun and the coverage gate remains required.
Keep this issue open to track recurrence and determine whether the artifact service or runner networking needs a more durable workaround.