Fix NaN handling and transparency in shape rendering

[timtreis]

• Fix KeyError in _convert_shapes by using proper DataFrame indexing
• Improve NaN value handling in color processing to use na_color correctly
• Preserve transparency from colormaps when fill_alpha is specified
• Add support for mixed numeric/color arrays in shape coloring
• Refactor _convert_shapes to use positional indexing for better reliability

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.41758% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.42%. Comparing base (491dae9) to head (d561e34).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/utils.py	84.90%	24 Missing ⚠️
src/spatialdata_plot/pl/render.py	65.21%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #503      +/-   ##
==========================================
+ Coverage   83.38%   84.42%   +1.03%     
==========================================
  Files           8        8              
  Lines        2106     2131      +25     
==========================================
+ Hits         1756     1799      +43     
+ Misses        350      332      -18     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	91.23% <65.21%> (-1.38%)	⬇️
src/spatialdata_plot/pl/utils.py	80.19% <84.90%> (+2.13%)	⬆️

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