Require prefix for the attach tag.

For `mjs_attachFrame` and `mjs_attachBody`, require either a prefix or suffix.

It is tempting for users to not use a prefix (see #2162), but this causes issues. For example, it creates a duplicate "main" default class in the parent as a child of the already existing "main" default.

PiperOrigin-RevId: 688535755
Change-Id: I653181ca35943f1e4eb89c70fca8b5289818324e

src/user/user_objects.cc

@@ -850,6 +850,10 @@ mjCBody& mjCBody::operator+=(const mjCFrame& other) {
   other.model->suffix = other.suffix;
   other.model->StoreKeyframes(model);
 
+  if (other.prefix.empty() && other.suffix.empty()) {
+    throw mjCError(this, "either prefix or suffix must be non-empty");
+  }
+
   // attach defaults
   if (other.model != model) {
     mjCDef* subdef = new mjCDef(*other.model->Default());
@@ -1812,6 +1816,10 @@ mjCFrame& mjCFrame::operator+=(const mjCBody& other) {
   other.model->prefix = "";
   other.model->suffix = "";
 
+  if (other.prefix.empty() && other.suffix.empty()) {
+    throw mjCError(this, "either prefix or suffix must be non-empty");
+  }
+
   mjCBody* subtree = new mjCBody(other, model);
   other.ForgetKeyframes();
   other.model->prefix = subtree->prefix;