Fix unused cmap in render_points

[melonora]

The test test_plot_coloring_with_cmap in the tests for points was wrongly passing. When switching from the use of get_categorical_obs to _get_categorical_color_mapping, the cmap was not anymore in the arguments. This led to the default colors always being used despite a colormap being defined. This PR fixes that issue by updating _get_categorical_color_mapping.
On main:

With this PR:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.21%. Comparing base (3b82c78) to head (0a0a81d).

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/utils.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #432      +/-   ##
==========================================
+ Coverage   85.20%   85.21%   +0.01%     
==========================================
  Files           8        8              
  Lines        1751     1759       +8     
==========================================
+ Hits         1492     1499       +7     
- Misses        259      260       +1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	96.89% <ø> (ø)
src/spatialdata_plot/pl/utils.py	78.34% <87.50%> (+0.07%)	⬆️

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