fix(walkthrough): keep unauthenticated models out of the picker

Treat an empty allowedProviderIds list as allow-none, disable Generate
when no usable model is selected, and mute the button styling so it
reads as unavailable rather than actionable.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/ui/src/components/model-picker/ModelPickerList.tsx

@@ -436,7 +436,10 @@ export const ModelPickerList: React.FC<ModelPickerListProps> = ({
   );
 
   const allowedProviderSet = React.useMemo(() => {
-    if (!allowedProviderIds || allowedProviderIds.length === 0) return null;
+    // undefined = no restriction; [] = allow none. Treating empty like
+    // "unrestricted" would resurface providers without a login in pickers that
+    // intentionally pass the authenticated-only list.
+    if (!allowedProviderIds) return null;
     return new Set(allowedProviderIds);
   }, [allowedProviderIds]);