Disable E741 in stub files

[calumy]

Summary

As noted in #10569, E741 should not be active in .pyi files as the file author does not control the variable name. Instead, this is deferred to the author of the package for which the stub file is substituting for.

My rust skills are being stretched here. I assume there is a more elegant way of implementing the check checker.source_type != PySourceType::Stub in the ambiguous_variable_name function so that it is only implemented in one place (similar to non_unique_enums); however, I can't get the borrow checker to agree to this.

Test Plan

Added a new test case were E741.py was duplicated and renamed to E741.pyi and a snapshot to confirm that no errors were raised for this new case as E741 should be disabled in .pyi files.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[AlexWaygood]

Thanks @calumy, this is great! I pushed a few small fixups to your PR:

1. I made the change preview-only for now; we can stabilise it in Ruff 0.7. Strictly speaking we could probably get away with doing this in a patch release, since it's a reduction in scope rather than an increase in scope -- but it's still a big change in scope, so I feel much safer doing it in preview only first :-)
2. I added some docs on how it works differently for .pyi files
3. I made it a bit more DRY by putting the check for whether it's a stub file in the ambiguous_variable_name() function itself, so we don't have to repeat it everywhere.

LMK if you'd like me to explain any of it, very happy to do so :-)

[calumy]

Thanks @calumy, this is great! I pushed a few small fixups to your PR:

1. I made the change preview-only for now; we can stabilise it in Ruff 0.7. Strictly speaking we could probably get away with doing this in a patch release, since it's a reduction in scope rather than an increase in scope -- but it's still a big change in scope, so I feel much safer doing it in preview only first :-)
2. I added some docs on how it works differently for .pyi files
3. I made it a bit more DRY by putting the check for whether it's a stub file in the ambiguous_variable_name() function itself, so we don't have to repeat it everywhere.

LMK if you'd like me to explain any of it, very happy to do so :-)

All makes sense to me; thanks for laying it out clearly.