Fix O(n^2) pd.concat in _get_cs_contents

[timtreis]

Summary

• _get_cs_contents was calling pd.concat inside a loop with four astype("bool") casts per iteration — O(n²) row copies for n coordinate systems. For a 49-sample Visium dataset this means 49 growing concat calls
• All callers used cs_contents.query(f"cs == '{cs}'") inside loops, which is O(n) per call and raises pandas.errors.UndefinedVariableError for coordinate system names containing single quotes (e.g. "patient's_sample")

Fixes:

• _get_cs_contents now collects rows as a list and builds the DataFrame once, with a single pass for astype("bool") — O(n) total
• All .query(f"cs == '{cs}'") usages replaced with cs_contents.set_index("cs").loc[cs] — O(1) lookup, no injection risk
• The redundant second call to _get_cs_contents in show() is eliminated by reusing the already-computed cs_index

Measured speedup (50 runs each):

n=  10:  16x faster
n=  50:  55x faster  
n= 100: 101x faster

Closes #602

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.39%. Comparing base (e1f8863) to head (6ffb163).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/utils.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #642   +/-   ##
=======================================
  Coverage   76.39%   76.39%           
=======================================
  Files          11       11           
  Lines        3237     3237           
  Branches      759      761    +2     
=======================================
  Hits         2473     2473           
  Misses        466      466           
  Partials      298      298           

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/basic.py	85.66% <100.00%> (+0.18%)	⬆️
src/spatialdata_plot/pl/utils.py	66.21% <80.00%> (-0.23%)	⬇️

... and 2 files with indirect coverage changes

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