Fix FutureWarning for na_action in Categorical.map

[timtreis]

Summary

• Fixes the FutureWarning from pandas ≥2.1 about the deprecated default na_action parameter in Categorical.map() by explicitly passing na_action="ignore"
• The mapper is always a dict (never a callable), so na_action has no effect on behavior — this is a pure warning suppression with zero functional change

Closes #540

Test plan

• Existing tests pass (no behavioral change)
• Verify warning is no longer emitted with pandas ≥2.1

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.96%. Comparing base (c538e7f) to head (c13b92f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #541   +/-   ##
=======================================
  Coverage   83.96%   83.96%           
=======================================
  Files           9        9           
  Lines        2595     2595           
=======================================
  Hits         2179     2179           
  Misses        416      416           

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/utils.py	79.30% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.