Apply prefix/suffix to flex material names.

PiperOrigin-RevId: 866518766
Change-Id: I5f042f52a27d2a6c66fe9642bccc73328fe14603

src/user/user_mesh.cc

@@ -4036,6 +4036,9 @@ void mjCFlex::NameSpace(const mjCModel* m) {
   for (auto& name : spec_nodebody_) {
     name = m->prefix + name + m->suffix;
   }
+  if (!spec_material_.empty() && model != m) {
+    spec_material_ = m->prefix + spec_material_ + m->suffix;
+  }
 }