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.h

@@ -29,7 +29,7 @@ class mjXWriter : public mjXBase {
  public:
   mjXWriter();                                        // constructor
   virtual ~mjXWriter() = default;                     // destructor
-  void SetModel(const mjSpec* _spec, const mjModel* m = nullptr);
+  void SetModel(mjSpec* _spec, const mjModel* m = nullptr);
 
   // write XML document to string
   std::string Write(char *error, std::size_t error_sz);