refactor(python): Refactor Series/DataFrame.__getitem__ logic

[stinodego]

This is in preparation for #4924

Changes

• Rename DataFrame.select_at_idx in the PyO3 bindings to DataFrame.to_series to match the Python public API.
• Update DataFrame.to_series to accept negative indices (implemented in the PyO3 bindings, just like Series.get_index_signed). Remove this bound checking from various other places.
• Address some edge cases:
  • Handle empty list as column selector
  • Handle boolean NumPy array as column selector
• Remove full scans of Sequence inputs (e.g. is_int_sequence), use first entry instead and fail later.
• Add lots of tests

We should probably add some methods to the API like DataFrame.filter_columns and DataFrame.get_columns_by_index - these are only available through __getitem__ right now.

[codecov]

Codecov Report

Attention: Patch coverage is 92.33871% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 81.51%. Comparing base (d265cd1) to head (05bc79d).

Files	Patch %	Lines
py-polars/polars/_utils/getitem.py	90.95%	11 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16482      +/-   ##
==========================================
- Coverage   81.51%   81.51%   -0.01%     
==========================================
  Files        1409     1410       +1     
  Lines      184974   185032      +58     
  Branches     2981     2981              
==========================================
+ Hits       150778   150823      +45     
- Misses      33675    33693      +18     
+ Partials      521      516       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.