Fix render_labels silently accepting fill_alpha=0 and outline_alpha=0

[timtreis]

Summary

• Fixes render_labels(fill_alpha=0, outline_alpha=0) silently renders invisible labels instead of erroring #630: render_labels(fill_alpha=0, outline_alpha=0) produced an empty plot (alpha=0 overlay) instead of erroring.
• The first branch of the dispatch chain matched fill_alpha == outline_alpha, routing (0, 0) to the fill-only path and leaving the existing else: raise ValueError(...) unreachable.
• Add an explicit guard before the dispatch chain so the documented error is actually raised.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.35%. Comparing base (072f04e) to head (65281f6).

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
- Coverage   76.39%   76.35%   -0.05%     
==========================================
  Files          11       11              
  Lines        3237     3239       +2     
  Branches      761      762       +1     
==========================================
  Hits         2473     2473              
- Misses        466      467       +1     
- Partials      298      299       +1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	86.41% <0.00%> (-0.26%)	⬇️

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