Raise ValueError on invalid scale in render_images

Previously, render_images(scale="...") silently reset an unknown scale
name to None and auto-picked a resolution, discarding the user's intent
without warning. The downstream _multiscale_to_spatial_image already had
the right ValueError but was unreachable because the value was cleared
beforehand. Validate at the entry point and surface the error with the
list of valid scales.

Closes #623

Author: timtreis

src/spatialdata_plot/pl/utils.py

@@ -3007,12 +3007,12 @@ def _validate_image_render_params(
         element_params[el]["norm"] = norm
         scale = param_dict["scale"]
         if scale and isinstance(param_dict["sdata"][el], DataTree):
-            if scale not in list(param_dict["sdata"][el].keys()) and scale != "full":
-                element_params[el]["scale"] = None
-            else:
-                element_params[el]["scale"] = scale
-        else:
-            element_params[el]["scale"] = scale
+            valid_scales = list(param_dict["sdata"][el].keys())
+            if scale not in valid_scales and scale != "full":
+                raise ValueError(
+                    f"Scale '{scale}' does not exist in image '{el}'. Valid scales: {valid_scales + ['full']}."
+                )
+        element_params[el]["scale"] = scale
         element_params[el]["colorbar"] = param_dict["colorbar"]
         element_params[el]["colorbar_params"] = param_dict["colorbar_params"]