fix(downloads): set 0o644 permissions on downloaded files

[smoparth]

NamedTemporaryFile creates files with mode 0o600, making them unreadable by external wheel servers (e.g. nginx) that run as a different user. This caused 403 Forbidden errors when uv pip install tried to fetch wheels from the local package index.

Use os.fchmod before close+rename to widen permissions to 0o644, preserving the atomicity and thread-safety benefits of NamedTemporaryFile.

Closes: #1281

Pull Request Description

What

Why

• PR follows CONTRIBUTING.md guidelines

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between e16b86b and efef444.

📒 Files selected for processing (2)

• src/fromager/downloads.py
• tests/test_downloads.py

📝 Walkthrough

Walkthrough

The download helper now imports stat and changes temporary file permissions to include group-read and other-read access before closing and atomically renaming the file. A new test mocks a download and verifies those permission bits on the resulting file.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The change satisfies #1281 by making downloaded wheels readable while preserving atomic rename behavior.
Out of Scope Changes check	✅ Passed	The PR stays focused on download permissions and the corresponding test, with no obvious unrelated changes.
Title check	✅ Passed	The title clearly summarizes the main change: widening downloaded file permissions to 0o644.
Description check	✅ Passed	The description matches the patch and explains the permission fix and its motivation.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@tests/test_downloads.py`:
- Around line 77-85: Update test_download_url_world_readable to assert that mode
equals exactly 0o644, replacing the separate group-read and other-read bit
checks while preserving the existing download setup.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

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Run ID: 64fcbe23-f5a5-43e3-87ea-935db549a8fe

📥 Commits

Reviewing files that changed from the base of the PR and between ebbd889 and e16b86b.

📒 Files selected for processing (2)

• src/fromager/downloads.py
• tests/test_downloads.py

[rd4398]

This looks good! Thanks

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