fix(show): render all coordinate systems for type-distributed elements

[timtreis]

Summary

Fixes #694.

_resolve_coordinate_systems computed elements_to_be_rendered from the leaked validation-loop variable cs — bound to the last coordinate system only. Because _get_elements_to_be_rendered keys on element type (has_images/has_labels/…), a CS lacking a queued element type was wrongly pruned by _get_valid_cs whenever elements of that type lived in another CS. For multi-CS plots with type-distributed elements, a coordinate system that should have rendered was silently dropped.

Fix

Union the elements across all coordinate systems before pruning (the comprehension scope also eliminates the leaked variable):

elements_to_be_rendered = list(
    dict.fromkeys(e for cs in coordinate_systems for e in _get_elements_to_be_rendered(render_cmds, cs_index, cs))
)

dict.fromkeys dedupes while preserving order.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.15%. Comparing base (4ba13a3) to head (9f8cdd2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #718      +/-   ##
==========================================
+ Coverage   77.13%   77.15%   +0.02%     
==========================================
  Files          14       14              
  Lines        4457     4457              
  Branches     1023     1023              
==========================================
+ Hits         3438     3439       +1     
  Misses        661      661              
+ Partials      358      357       -1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/basic.py	82.60% <100.00%> (+0.24%)	⬆️

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