fix: widen Variable.sum dim parameter type to DimsLike

[manud99]

The dim argument in Variable.sum has a too strict type declaration. It also accepts Iterable[Hashable] because it passes the dim argument to BaseExpression.sum. I adapted the type of Variable.sum to fix this imprecision.

Changes proposed in this Pull Request

• fix: change Variable.sum method parameter type to DimsLike

Checklist

• AI-generated content is marked (see AGENTS.md).
• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[codspeed-hq]

Merging this PR will not alter performance

✅ 173 untouched benchmarks
⏩ 173 skipped benchmarks¹

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_{Comparing manud99:patch-1 (f8fd994) with master (f759df9)}

Footnotes

1. 173 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[FBumann]

@manud99 Thanks for this quick quality fix.