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.h

@@ -30,6 +30,6 @@ mjSpec* ParseSpecFromString(std::string_view xml, const mjVFS* vfs = nullptr,
                             char* error = nullptr, int nerror = 0);
 
 // Main writer function
-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);
 
 #endif  // MUJOCO_SRC_XML_XML_H_