Fix IndexError in _build_datashader_color_key and restore _hex_no_alpha

- Convert color_vector to numpy array before indexing to avoid
  IndexError when color_vector is shorter than cat_series (fixes CI
  failure in test_color_key_unseen_category_gets_na_color)
- Guard against out-of-bounds index when unique code appears beyond
  the color_vector length
- Revert inline c[:7] slice back to _hex_no_alpha for correctness
  (the slice silently mangles non-hex 9-char strings)
- Use explicit `is not None` check for col_for_color guard

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/_datashader.py

@@ -61,10 +61,11 @@ def _build_datashader_color_key(
 ) -> dict[str, str]:
     """Build a datashader ``color_key`` dict from a categorical series and its color vector."""
     na_hex = _hex_no_alpha(na_color_hex) if na_color_hex.startswith("#") else na_color_hex
+    colors_arr = np.asarray(color_vector, dtype=object)
     categories = np.asarray(cat_series.categories, dtype=str)
     codes = np.asarray(cat_series.codes)
 
-    if len(color_vector) != len(codes):
+    if len(colors_arr) != len(codes):
         logger.warning(
             f"color_vector length ({len(color_vector)}) does not match categorical series length "
             f"({len(codes)}); some categories may receive the na_color fallback."
@@ -76,9 +77,9 @@ def _build_datashader_color_key(
 
     first_color: dict[str, str] = {}
     for code, idx in zip(unique_codes, first_indices, strict=True):
-        if code < 0:
+        if code < 0 or idx >= len(colors_arr):
             continue
-        c = color_vector[idx]
+        c = colors_arr[idx]
         first_color[categories[code]] = _hex_no_alpha(c) if isinstance(c, str) and c.startswith("#") else c
 
     return {cat: first_color.get(cat, na_hex) for cat in categories}

src/spatialdata_plot/pl/render.py

@@ -786,7 +786,7 @@ def _render_points(
 
     # When color was already loaded from a table (line 690), pass it directly
     # to avoid a redundant get_values() call inside _set_color_source_vec.
-    _preloaded = points_pd_with_color[col_for_color] if added_color_from_table and col_for_color else None
+    _preloaded = points_pd_with_color[col_for_color] if added_color_from_table and col_for_color is not None else None
 
     color_source_vector, color_vector, _ = _set_color_source_vec(
         sdata=sdata_filt,
@@ -926,7 +926,7 @@ def _render_points(
             and isinstance(color_vector[0], str)
             and color_vector[0].startswith("#")
         ):
-            color_vector = np.asarray([c[:7] if len(c) == 9 else c for c in color_vector])
+            color_vector = np.asarray([_hex_no_alpha(c) for c in color_vector])
 
         nan_shaded = None
         if color_by_categorical or col_for_color is None: