fix: Handle slice pushdown in PythonUDF GroupBy

[coastalwhite]

Fixes #20002.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 59.00%. Comparing base (d86e44b) to head (0104e82).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/frame/group_by/mod.rs	0.00%	8 Missing ⚠️
crates/polars-mem-engine/src/executors/group_by.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #20132       +/-   ##
===========================================
- Coverage   79.52%   59.00%   -20.52%     
===========================================
  Files        1563     1563               
  Lines      217186   217203       +17     
  Branches     2465     2465               
===========================================
- Hits       172715   128170    -44545     
- Misses      43910    88472    +44562     
  Partials      561      561               

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