Extend make_palette_from_data to support labels elements #662

Summary

sdp.pl.make_palette_from_data(sdata, element, color, ...) currently raises KeyError when element is a labels (raster) element:

KeyError: "Element 'blobs_labels' not found in sdata.shapes or sdata.points.
Available elements: [...]. Note: labels (raster) elements are not yet supported."

Source: _resolve_element in src/spatialdata_plot/pl/_palette.py:278–306. The error message itself flags this as a TODO ("not yet supported"), so this is a known gap rather than a principled exclusion.

Why it's worth doing

• render_labels accepts palette=, so users naturally expect make_palette_from_data to produce one for labels too.
• For labels, the categorical column lives on the linked AnnData table (adata.obs[<col>]) — this is actually simpler than the current shapes/points path, which checks the GeoDataFrame first and only falls back to the table.
• Without it, users get a clean {category: hex} dict for shapes/points but have to hand-roll one for labels. The asymmetry is surprising and breaks the "one helper for any categorical" mental model.
• It bites the upcoming color/palette tutorial notebook (Adding plotting specific documentation #463): the most natural example (categorical coloring of segmentation masks) doesn't work; we currently demo it on polygons instead.

Proposed scope

1. Extend _resolve_element (src/spatialdata_plot/pl/_palette.py:278) to handle labels: when element is in sdata.labels, skip the GeoDataFrame branch and resolve the column from the linked table directly.
2. Reuse table_name argument resolution analogous to _validate_col_for_column_table (src/spatialdata_plot/pl/utils.py:3155–3163) for the multi-table-annotates-the-same-element case.
3. Update the make_palette_from_data docstring to drop the "shapes or points" restriction.
4. Add a regression test mirroring the existing shapes/points coverage (a labels element with a categorical column in the linked table → expected {category: hex} dict).

Estimated diff: ~30–50 LOC plus tests. No public API change; only widens an existing function.

Open questions

• Should NaN values in the categorical column produce a na_color key in the returned dict, or be omitted? Match whatever render_labels(palette=..., na_color=...) does today.
• For labels annotated by multiple tables without table_name= set, raise the same error as the shapes/points path (don't pick silently).

Filed alongside #463 (plotting documentation) — the new color tutorial currently sidesteps this by using blobs_polygons instead of blobs_labels for the §5 demo.