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.cc

@@ -395,7 +395,7 @@ mjSpec* ParseSpecFromString(std::string_view xml, const mjVFS* vfs, char* error,
 }
 
 // Main writer function - calls mjXWrite
-std::string WriteXML(const mjModel* m, const mjSpec* spec, char* error, int nerror) {
+std::string WriteXML(const mjModel* m, mjSpec* spec, char* error, int nerror) {
   LocaleOverride locale_override;
 
   // check for empty model