[refurb] Add fix safety section (FURB129)

[Kalmaegi]

Summary

This PR add the fix safety section for rule FURB129 in readlines_in_for.rs for #15584

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[dylwil3]

Thanks for looking into this! I don't think I understand the reasoning you provided - both readlines and directly iterating will consume the stream.

In fact I think this fix should actually be marked as safe , so I'm closing in favor of #17644 (see there for a justification of why I think it ought to be safe).