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/user/user_api.h

@@ -66,6 +66,9 @@ MJAPI int mjs_activatePlugin(mjSpec* s, const char* name);
 // Turn deep copy on or off attach. Returns 0 on success.
 MJAPI int mjs_setDeepCopy(mjSpec* s, int deepcopy);
 
+// Copy real-valued arrays from model to spec, returns 1 on success.
+MJAPI int mj_copyBack(mjSpec* s, const mjModel* m);
+
 
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