Fix render_shapes losing transformation after groups filtering

[timtreis]

Summary

• Fixes Plotting cluster sometimes results in coordinate system not being found, even if set correctly #420 (and hardens against render_shapes() fails with datashader when the coordinate system is not global #447 regression)
• Moves _prepare_transformation() call in _render_shapes() to before the groups filtering block, so the coordinate-system transformation is captured while the element's .attrs metadata is still intact
• The GeoDataFrame() re-wrap that follows groups filtering strips .attrs, which would silently lose the transformation if read afterwards

Root cause

In _render_shapes, the transformation was read after groups filtering and a gpd.GeoDataFrame() re-wrap that strips .attrs. With current geopandas/pandas, .attrs happen to survive boolean indexing + reset_index(), but this is documented as experimental. The GeoDataFrame() constructor definitively strips them. Moving the read earlier makes the code robust regardless of pandas .attrs propagation behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.06%. Comparing base (6b2c484) to head (46b80ce).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #564   +/-   ##
=======================================
  Coverage   74.06%   74.06%           
=======================================
  Files          10       10           
  Lines        2826     2826           
  Branches      651      651           
=======================================
  Hits         2093     2093           
  Misses        457      457           
  Partials      276      276           

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	82.84% <100.00%> (ø)

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