Fix cmap validation comparing against full image channels instead of selected

When user specifies `channel=[0]` on a 3-channel image with
`cmap=['gray']`, the cmap matched the selected channel count but was
nullified because it didn't match the full image channel count. Now
the comparison uses the selected channel count (or full count when no
channel is specified).

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

Author: timtreis

src/spatialdata_plot/pl/utils.py

@@ -2867,10 +2867,10 @@ def _validate_image_render_params(
 
         cmap = param_dict["cmap"]
         if cmap is not None:
+            expected_len = len(channel) if channel is not None else len(spatial_element_ch)
             if len(cmap) == 1:
-                cmap_length = len(channel) if channel is not None else len(spatial_element_ch)
-                cmap = cmap * cmap_length
-            if (channel is not None and len(cmap) != len(channel)) or len(cmap) != len(spatial_element_ch):
+                cmap = cmap * expected_len
+            if len(cmap) != expected_len:
                 cmap = None
         element_params[el]["cmap"] = cmap
         element_params[el]["norm"] = param_dict["norm"]