Fix points groups filtering: also filter adata, add baseline images

- Filter adata alongside points/color_vector so matplotlib scatter
  gets matching x/y and c array lengths
- Add baseline reference images for groups+na_color=None tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/render.py

@@ -853,8 +853,9 @@ def _render_points(
             if not hasattr(color_vector, "reset_index")
             else (color_vector[keep].reset_index(drop=True))
         )
-        # filter the materialized points and re-register in sdata_filt
+        # filter the materialized points, adata, and re-register in sdata_filt
         points = points[keep].reset_index(drop=True)
+        adata = adata[keep].copy()
         points_dd = dask.dataframe.from_pandas(points, npartitions=1)
         sdata_filt.points[element] = PointsModel.parse(points_dd, coordinates={"x": "x", "y": "y"})
         set_transformation(