fix(python): Allow list argument in group_by().map_groups()

[stakeswky]

Fixes #26672

Summary

df.group_by(['a', 'b']).map_groups(lambda df: df) raises TypeError: cannot call map_groups when grouping by an expression, even though all elements are plain column name strings.

Root Cause

group_by accepts *by (variadic), so df.group_by(['a', 'b']) stores self.by = (['a', 'b'],). The check isinstance(c, str) for c in self.by iterates over the outer tuple and finds a list, not a string.

Fix

Flatten self.by via _parse_inputs_as_iterable (already imported and used elsewhere in the module) before the string check. This is consistent with how other methods handle the variadic *by parameter.

As @ritchie46 noted in the issue, the check was overly strict — a list of strings is a perfectly valid input.

Test

Added test_map_groups_group_by_list_26672 verifying both group_by(['a', 'b']) and group_by('a', 'b') work with map_groups.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.66%. Comparing base (9d60226) to head (65ac835).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/src/polars/dataframe/group_by.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26707      +/-   ##
==========================================
+ Coverage   81.00%   81.66%   +0.66%     
==========================================
  Files        1805     1805              
  Lines      248021   248090      +69     
  Branches     3132     3131       -1     
==========================================
+ Hits       200902   202612    +1710     
+ Misses      46313    44673    -1640     
+ Partials      806      805       -1     

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