Parameter palette can now be used without specifying groups

[SAY-5]

Closes #605.

_validate_render_params raised ValueError: When specifying 'palette', 'groups' must also be specified whenever palette=list was supplied without groups=. The dict path has no such restriction, so a list of three colors against three categories forced users to redundantly re-list every category just to satisfy validation.

Dropped the groups is None requirement; the existing length check at _get_colors_for_categorical_obs already raises if the list does not match the category count, and the natural-order mapping there now applies. Regression test in test_render_shapes.py asserts the legend matches the underlying categories.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.48%. Comparing base (53abe71) to head (ccb5905).

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/utils.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #659      +/-   ##
==========================================
- Coverage   77.50%   77.48%   -0.02%     
==========================================
  Files          11       11              
  Lines        3569     3567       -2     
  Branches      837      836       -1     
==========================================
- Hits         2766     2764       -2     
+ Misses        481      480       -1     
- Partials      322      323       +1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/utils.py	67.58% <0.00%> (+0.01%)	⬆️

... and 1 file with indirect coverage changes

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[timtreis]

Hey @SAY-5, thank you for your contribution.