Copy UIDs during mj_copySpec.

This will produce the same mjSpec signature after a deep copy.

PiperOrigin-RevId: 743538018
Change-Id: Ic0f5b06a00e8883cb92dc98159db3bea02cb7a36

test/user/user_api_test.cc

@@ -538,6 +538,9 @@ TEST_F(PluginTest, RecompileCompare) {
         // copy spec
         mjSpec* s_copy = mj_copySpec(s);
 
+        // compare signature
+        EXPECT_EQ(s->element->signature, s_copy->element->signature) << xml;
+
         // compile twice and compare
         mjModel* m_old = mj_compile(s, nullptr);