Raise ValueError on invalid scale in render_images

[timtreis]

Summary

• render_images(scale=...) silently reset unknown scale names to None and auto-picked a resolution, discarding the user's intent.
• The downstream _multiscale_to_spatial_image already had the right ValueError but was unreachable because _validate_image_render_params cleared the value beforehand.
• Validate at the entry point and surface the error with the list of valid scales (including "full").

Closes #623

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.07%. Comparing base (29b1cc1) to head (b0c1cde).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/utils.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #654   +/-   ##
=======================================
  Coverage   77.07%   77.07%           
=======================================
  Files          11       11           
  Lines        3276     3276           
  Branches      772      772           
=======================================
  Hits         2525     2525           
  Misses        453      453           
  Partials      298      298           

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/utils.py	67.31% <50.00%> (ø)

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