fix: Fix map_elements for List return dtypes

[stinodego]

Closes #18472

This makes sure we respect the return_dtype if it's a List type, rather than discarding it and relying on auto-inference.

[codecov]

Codecov Report

Attention: Patch coverage is 63.93443% with 22 lines in your changes missing coverage. Please review.

Project coverage is 79.83%. Comparing base (6d4b79d) to head (94dd259).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-python/src/map/series.rs	57.14%	15 Missing ⚠️
crates/polars-python/src/series/map.rs	66.66%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18567      +/-   ##
==========================================
+ Coverage   79.80%   79.83%   +0.03%     
==========================================
  Files        1501     1501              
  Lines      201952   201736     -216     
  Branches     2868     2869       +1     
==========================================
- Hits       161163   161063     -100     
+ Misses      40243    40127     -116     
  Partials      546      546              

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