Do not self-attach custom tags.

Fixes #2162.

PiperOrigin-RevId: 688518165
Change-Id: Iad90e159514c96635676a8dd53fca41e3998d8fc

src/user/user_model.cc

@@ -386,6 +386,8 @@ mjCModel& mjCModel::operator+=(const mjCModel& other) {
     for (const auto& key : other.key_pending_) {
       key_pending_.push_back(key);
     }
+    CopyList(numerics_, other.numerics_);
+    CopyList(texts_, other.texts_);
   }
   CopyList(flexes_, other.flexes_);
   CopyList(pairs_, other.pairs_);
@@ -394,8 +396,6 @@ mjCModel& mjCModel::operator+=(const mjCModel& other) {
   CopyList(equalities_, other.equalities_);
   CopyList(actuators_, other.actuators_);
   CopyList(sensors_, other.sensors_);
-  CopyList(numerics_, other.numerics_);
-  CopyList(texts_, other.texts_);
   CopyList(tuples_, other.tuples_);
 
   // create new plugins and map them

test/user/user_api_test.cc

@@ -631,6 +631,10 @@ static constexpr char xml_child[] = R"(
       </default>
     </default>
 
+    <custom>
+      <numeric data="10" name="constant"/>
+    </custom>
+
     <asset>
       <texture name="texture" type="2d" builtin="checker" width="32" height="32"/>
       <material name="material" texture="texture" texrepeat="1 1" texuniform="true"/>
@@ -694,6 +698,10 @@ TEST_F(MujocoTest, AttachSame) {
       </default>
     </default>
 
+    <custom>
+      <numeric data="10" name="constant"/>
+    </custom>
+
     <asset>
       <texture name="texture" type="2d" builtin="checker" width="32" height="32"/>
       <material name="material" texture="texture" texrepeat="1 1" texuniform="true"/>
@@ -833,6 +841,10 @@ TEST_F(MujocoTest, AttachDifferent) {
       </default>
     </default>
 
+    <custom>
+      <numeric data="10" name="attached-constant-1"/>
+    </custom>
+
     <asset>
       <texture name="attached-texture-1" type="2d" builtin="checker" width="32" height="32"/>
       <material name="attached-material-1" texture="attached-texture-1" texrepeat="1 1" texuniform="true"/>
@@ -957,6 +969,10 @@ TEST_F(MujocoTest, AttachFrame) {
       </default>
     </default>
 
+    <custom>
+      <numeric data="10" name="attached-constant-1"/>
+    </custom>
+
     <asset>
       <texture name="attached-texture-1" type="2d" builtin="checker" width="32" height="32"/>
       <material name="attached-material-1" texture="attached-texture-1" texrepeat="1 1" texuniform="true"/>
@@ -1063,6 +1079,10 @@ void TestDetachBody(bool compile) {
       <material name="material" texture="texture" texrepeat="1 1" texuniform="true"/>
     </asset>
 
+    <custom>
+      <numeric data="10" name="constant"/>
+    </custom>
+
     <worldbody>
       <frame name="pframe">
         <frame name="cframe">