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/xml/xml_native_writer.cc

@@ -885,12 +885,12 @@ mjXWriter::mjXWriter(void) {
 
 
 // cast model
-void mjXWriter::SetModel(const mjSpec* _spec, const mjModel* m) {
+void mjXWriter::SetModel(mjSpec* _spec, const mjModel* m) {
   if (_spec) {
     model = static_cast<mjCModel*>(_spec->element);
   }
   if (m) {
-    model->CopyBack(m);
+    mj_copyBack(&model->spec, m);
   }
 }