fix: update Polars selector type annotation to use `Selector`

[schmidma]

Summary

This PR fixes a type annotation issue with Polars selectors. Polars introduced a new Selector class. As a result, the current import of _selector_proxy_ in great-tables fails when using at least version 1.32.0 of Polars.

Without this change, type-checking great-tables with recent Polars versions fails due to missing _selector_proxy_.

Changes

• Replaced _selector_proxy_ with Selector
• Updated type alias: PlSelectExpr = Selector

Notes

• This is a type-checking only change; no runtime behavior is affected.
• Verified with latest Polars release where Selector is available.

Related GitHub Issues and PRs

• https://github.com/pola-rs/polars/pull/23351

Checklist

• [x] I understand and agree to the Code of Conduct.
• [x] I have followed the Style Guide for Python Code as best as possible for the submitted code.
• ~~[ ] I have added pytest unit tests for any new functionality.~~

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 91.45%. Comparing base (654757a) to head (24dcd96).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #770   +/-   ##
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  Coverage   91.45%   91.45%           
=======================================
  Files          47       47           
  Lines        5558     5558           
=======================================
  Hits         5083     5083           
  Misses        475      475           

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[jrycw]

@rich-iannone and @machow, perhaps we should use a try-import here to handle the compatibility issue.