fix(color): honour palette for outline-only columns and over .uns colors

[timtreis]

Problem

Two ways an explicit palette was silently ignored:

1. Outline color palette is ignored in render_shapes if no color is passed #777 — render_shapes(outline_color="celltype", palette={...}, fill_alpha=0) (no color=) outlined with the default scanpy palette instead of the dict. Same for render_labels.
2. Labels dict palette — render_labels(color="ct", palette={"a": ..., "b": ...}) always rendered default colors. Shapes/points had the same issue once <col>_colors existed in the table's .uns.

Fixes #777.

Root cause

1. _gate_palette_and_groups (_validate.py) kept palette only when color= was a column. It predates outline-by-column (Allow outline_color to accept an obs column #683) and was never updated, so the outline lookup got palette=None.
2. _set_color_source_vec (_color.py) preferred .uns[<col>_colors] over a dict palette. render_labels always writes default colors into .uns before resolving (_maybe_set_label_colors), so a dict palette could never win for labels; for shapes/points it lost as soon as .uns colors were present (e.g. from an earlier plot).

Fix

1. Keep palette when a fill or outline column is set. groups stays tied to the fill column (the outline lookup never uses it).
2. Skip the .uns lookup when palette is a dict. A list palette with groups already overrode .uns colors (test_plot_respects_custom_colors_from_uns_with_groups_and_palette), so the dict palette now behaves the same way.

Behaviour matrix before → after (color="ct", dict palette {a: #ff00ff, b: #00ff00}):

element	.uns colors present	before	after
shapes	no	palette ✅	palette ✅
shapes	yes	.uns ❌	palette ✅
labels	no	defaults ❌	palette ✅
labels	yes	.uns ❌	palette ✅

List/str/no palette rows are unchanged.

Tests

• test_render_shapes.py::test_outline_color_column_respects_palette_without_fill_column — outline edge colors match the dict (Outline color palette is ignored in render_shapes if no color is passed #777).
• test_render_labels.py::test_render_labels_respects_dict_palette — rendered label pixels use the dict colors.

Both fail on main and pass here. Non-visual suite: 582 passed, 1 skipped.

Expected visual baseline changes

PaletteVisual_dict_palette_hex_labels.png and PaletteVisual_dict_palette_named_colors_labels.png currently show tab10 blue although the tests pass palette={"blobs_labels": "#E69F00"} / "coral". The baselines captured the bug; they'll be regenerated from CI artifacts in a follow-up commit.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.12%. Comparing base (956eb0c) to head (502a386).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #778      +/-   ##
==========================================
+ Coverage   80.09%   80.12%   +0.03%     
==========================================
  Files          18       18              
  Lines        4863     4866       +3     
  Branches     1080     1080              
==========================================
+ Hits         3895     3899       +4     
  Misses        603      603              
+ Partials      365      364       -1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/_color.py	70.50% <ø> (+0.15%)	⬆️
src/spatialdata_plot/pl/_validate.py	71.73% <100.00%> (+0.03%)	⬆️

... and 1 file with indirect coverage changes

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