Outline color palette is ignored in `render_shapes` if no color is passed

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When coloring shapes by a categorical column using only outline_color= (with fill_alpha=0), the palette dict is silently ignored and matplotlib's default tab10 colors are used instead. Passing the same column/palette via color= works correctly.

Environment: spatialdata 0.8.0, spatialdata-plot 0.4.2
Expected: outline_color= should resolve palette the same way color= does.
Workaround: pass the column to both color= and outline_color= (keeping fill_alpha=0).

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
import anndata as ad
import spatialdata as sd
import spatialdata_plot
from spatialdata.models import TableModel

sdata = sd.datasets.blobs()
shapes_key = "blobs_polygons"  # not annotated by the default blobs table
n = sdata[shapes_key].shape[0]

palette = {
    "EL": "#cba02c", "ES": "#ebc84f", "RL": "#0272b2", "RM": "#5799d1", "RS": "#9dcbec",
}
labels = pd.Categorical(list(palette.keys())[:n], categories=list(palette.keys()))

# Table annotating blobs_polygons with our categorical color column
obs = pd.DataFrame({"instance_id": np.arange(n), "region": pd.Categorical(["blobs_polygons"] * n)})
obs.index = obs.index.astype(str)
table = TableModel.parse(
    ad.AnnData(X=np.zeros((n, 1)), obs=obs),
    region=["blobs_polygons"], region_key="region", instance_key="instance_id",
)
sdata.tables["t"] = table
sdata["t"].obs["celltype"] = labels

# Works: palette correctly maps to fill colors
fig, ax = plt.subplots()
sdata.pl.render_shapes(shapes_key, color="celltype", palette=palette, table_name="t").pl.show(ax=ax)
fig.savefig("color_works.png", dpi=100)

# Bug: palette is ignored, falls back to default tab10 colors
fig, ax = plt.subplots()
sdata.pl.render_shapes(
    shapes_key, outline_color="celltype", palette=palette, fill_alpha=0,
    outline_width=1.5, table_name="t",
).pl.show(ax=ax)
fig.savefig("outline_color_bug.png", dpi=100)

Versions

render_shapes エントリポイントから開始し、color= と outline_color= および fill_alpha=0 を比較しながら、提供されている再現手順を実行します。各引数がカテゴリカルパレットをどのように解決するかを追跡します。完了条件は、outline_color= がデフォルトの tab10 カラーではなく指定されたパレットを使用し、既存の color= の動作が変わらないことです。