Resize keyframes also at the start of compilation.

Previously, it was assumed that attaching was the last operation performed on the mjSpec, so keyframes resulted in the incorrect size. In order to prevent information loss, we now only allow the resize function to expand the keyframe array, since attach should never remove a degree of freedom.

PiperOrigin-RevId: 804917828
Change-Id: Icfacd207ced1c6aac600d514f8d9065ceff2d347

test/user/user_objects_test.cc

@@ -464,7 +464,7 @@ TEST_F(KeyframeTest, BadSize) {
   size_t error_sz = 1024;
   mjModel* model = LoadModelFromString(xml, error, error_sz);
   EXPECT_THAT(model, IsNull());
-  EXPECT_THAT(error, HasSubstr("invalid qpos size, expected length 0"));
+  EXPECT_THAT(error, HasSubstr("invalid qpos size, expected 0, got 1"));
 }
 
 // ------------- test relative frame sensor compilation-------------------------