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

include/mujoco/mujoco.h

@@ -107,6 +107,9 @@ MJAPI mjSpec* mj_parseXMLString(const char* xml, const mjVFS* vfs, char* error,
 // Compile spec to model.
 MJAPI mjModel* mj_compile(mjSpec* s, const mjVFS* vfs);
 
+// Copy real-valued arrays from model to spec, returns 1 on success.
+MJAPI int mj_copyBack(mjSpec* s, const mjModel* m);
+
 // Recompile spec to model, preserving the state, return 0 on success.
 MJAPI int mj_recompile(mjSpec* s, const mjVFS* vfs, mjModel* m, mjData* d);