Fix upload_pypi: artifact name mismatch causes zero downloads

[Copilot]

The upload_pypi job was failing because download-artifact used pattern: artifact_*, which doesn't match the artifact actually uploaded under the name artifact — resulting in 0 artifacts downloaded and a PyPI publish failure.

Changes

• .github/workflows/release.yml: Replace pattern: artifact_* + merge-multiple: true with name: artifact in the download-artifact step, so it correctly resolves the artifact produced by build_wheel_and_sdist

# Before
- uses: actions/download-artifact@v8
  with:
    pattern: artifact_*       # never matches "artifact"
    merge-multiple: true
    path: dist

# After
- uses: actions/download-artifact@v8
  with:
    name: artifact            # exact match
    path: dist

[RobinDavid]

LGTM, thanks @copilot :)