fix(python): Handle nested categoricals in assert_series_equal when categorical_as_str=True

[stinodego]

Closes #16196

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.47%. Comparing base (c506d1f) to head (bd063e7).
Report is 1 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/testing/asserts/series.py	85.71%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16700   +/-   ##
=======================================
  Coverage   81.47%   81.47%           
=======================================
  Files        1416     1416           
  Lines      186889   186922   +33     
  Branches     3021     3029    +8     
=======================================
+ Hits       152259   152298   +39     
+ Misses      34097    34090    -7     
- Partials      533      534    +1     

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