Add mj_copyBack for copying real-valued arrays from mjModel back to mjSpec.

Also add `SaveAndReadXML` function to test fixture using `mjSpec` as input.

PiperOrigin-RevId: 765393821
Change-Id: Ic257addd2fc89678fc11b226c168c077626cc51e

src/user/user_api.cc

@@ -375,6 +375,14 @@ int mjs_setDeepCopy(mjSpec* s, int deepcopy) {
 
 
 
+// copy real-valued arrays from model to spec, returns 1 on success
+int mj_copyBack(mjSpec* s, const mjModel* m) {
+  mjCModel* model = static_cast<mjCModel*>(s->element);
+  return model->CopyBack(m);
+}
+
+
+
 // delete object, return 0 on success
 int mjs_delete(mjsElement* element) {
   mjCModel* model;

src/user/user_api.h

@@ -66,6 +66,9 @@ MJAPI int mjs_activatePlugin(mjSpec* s, const char* name);
 // Turn deep copy on or off attach. Returns 0 on success.
 MJAPI int mjs_setDeepCopy(mjSpec* s, int deepcopy);
 
+// Copy real-valued arrays from model to spec, returns 1 on success.
+MJAPI int mj_copyBack(mjSpec* s, const mjModel* m);
+
 
 //---------------------------------- Attachment ----------------------------------------------------
 

src/user/user_model.cc

@@ -4759,6 +4759,11 @@ bool mjCModel::CopyBack(const mjModel* m) {
     return false;
   }
 
+  if (spec.element->signature != m->signature) {
+    errInfo = mjCError(0, "incompatible signatures in CopyBack");
+    return false;
+  }
+
   // option and visual
   option = m->opt;
   visual = m->vis;