Fix categorical colors wrongly assigned to points with non-sequential index

[timtreis]

Summary

• Fixes Categorical colors wrongly assigned to points #358: when points have a non-sequential index (e.g. from .sample() or .subset()), categorical colors were assigned based on sorted index order instead of actual row order, causing colors to appear shuffled.
• Root cause: _reparse_points() round-trips through PointsModel.parse which sorts by index, but adata.X retained the original unsorted order. get_values() then returned colors in sorted order — misaligned with coordinates.
• Fix: reset the points index to sequential before adata construction and reparsing so both share the same positional order.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.61%. Comparing base (55d59b7) to head (d3ca5ba).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #570      +/-   ##
==========================================
+ Coverage   75.39%   75.61%   +0.22%     
==========================================
  Files          10       10              
  Lines        2906     2908       +2     
  Branches      672      672              
==========================================
+ Hits         2191     2199       +8     
+ Misses        431      428       -3     
+ Partials      284      281       -3     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	86.85% <100.00%> (+0.02%)	⬆️

... and 2 files with indirect coverage changes

🚀 New features to boost your workflow:

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