Fix Mesh PhysicsCollisionAPI by accounting for how it's referenced/instanced.

Because we introduce an additional parent scope when referencing a `Mesh` prim (required for instancing), we need to create an `over` prim that allows to actually operate on the to-be-referenced Mesh.

PiperOrigin-RevId: 753941355
Change-Id: I829be9e82fe650802afcbc46f4b2f4c81eb53bd1

src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc

@@ -66,7 +66,6 @@ using TfStaticData = pxr::TfStaticData<T>;
 
 // clang-format off
 TF_DEFINE_PRIVATE_TOKENS(kTokens,
-                         // Xform ops
                          ((body, "Body"))
                          ((body_name, "mujoco:body_name"))
                          ((geom, "Geom"))
@@ -89,7 +88,8 @@ TF_DEFINE_PRIVATE_TOKENS(kTokens,
                          ((outputsRgb, "outputs:rgb"))
                          ((inputsMetallic, "inputs:metallic"))
                          (repeat)
-                         );
+                         ((sourceMesh, pxr::UsdGeomTokens->Mesh))
+                        );
 
 // Using to satisfy TF_REGISTRY_FUNCTION macro below and avoid operating in PXR_NS.
 using pxr::TfEnum;
@@ -330,7 +330,7 @@ class ModelWriter {
     pxr::SdfPath subcomponent_path =
         CreatePrimSpec(data_, parent_path, name, pxr::UsdGeomTokens->Xform);
     pxr::SdfPath mesh_path =
-        CreatePrimSpec(data_, subcomponent_path, pxr::UsdGeomTokens->Mesh,
+        CreatePrimSpec(data_, subcomponent_path, kTokens->sourceMesh,
                        pxr::UsdGeomTokens->Mesh);
     mesh_paths_[*mesh->name] = subcomponent_path;
 
@@ -600,7 +600,15 @@ class ModelWriter {
     // Reference the mesh asset written in WriteMeshes.
     AddPrimReference(data_, subcomponent_path, mesh_paths_[*geom->meshname]);
 
-    return subcomponent_path;
+    // We want to use instancing with meshes, and it requires creating a parent
+    // scope to be referenced, with the Mesh prim as a child.
+    // To be able to actually manipulate the Mesh prim, we need to create and
+    // return the corresponding `over` prim as a child of the referencing prim.
+    pxr::SdfPath over_mesh_path =
+        CreatePrimSpec(data_, subcomponent_path, kTokens->sourceMesh,
+                       pxr::UsdGeomTokens->Mesh, pxr::SdfSpecifierOver);
+
+    return over_mesh_path;
   }
 
   pxr::SdfPath WriteSiteGeom(const mjsSite *site,