fix: insert properly from marketplace (#4725)

Fixes #4723

We removed body from template data but didn't fix roots. Later insertion
logic started to rely on these roots.

apps/builder/app/builder/features/marketplace/templates.tsx

@@ -45,9 +45,16 @@ const insertSection = ({
   instanceId: string;
 }) => {
   const fragment = extractWebstudioFragment(data, instanceId);
-  fragment.instances = fragment.instances.filter(
-    (instance) => instance.component !== "Body"
+  const body = fragment.instances.find(
+    (instance) => instance.component === "Body"
   );
+  // remove body and use its children as root insrances
+  if (body) {
+    fragment.instances = fragment.instances.filter(
+      (instance) => instance.component !== "Body"
+    );
+    fragment.children = body.children;
+  }
   const insertable = findClosestInsertable(fragment);
   if (insertable) {
     insertWebstudioFragmentAt(fragment, insertable);