🧹 [ci] Fix GitHub Actions download-artifact version

[Tcode-Motion]

🎯 What (the issue addressed):
Replaced the non-existent version v8 of actions/download-artifact with the valid version v4 in .github/workflows/pypi-publish.yml and .github/workflows/release.yml.

💡 Why (maintainability improvement):
The incorrect action version caused GitHub Actions to fail to resolve the artifact download step. Updating it to a valid major version (v4) restores the workflow execution.

✅ Verification (how safety was confirmed):
Verified the changes via grep on the affected workflows. Also executed cargo test --workspace to ensure no other code regressions were introduced.

✨ Result (improvement achieved):
The continuous integration pipelines for packaging and releasing are now configured correctly to download artifacts.

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PR created automatically by Jules for task 7308680668574039603 started by @Tcode-Motion

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