Fix trilinear flexcomp with unused vertices.

Fix a crash and dimension mismatch issue in user_flexcomp when dealing with interpolated="true" meshes that contain unused vertices. Previously, vertbody would correspond only to used vertices while vert included all vertices, leading to assertions or out-of-bounds access. Also enable node pinning for interpolated meshes.

PiperOrigin-RevId: 865881995
Change-Id: Id150f00f22b5f48896ffbffddf03c57b252de8c0

src/user/user_mesh.cc

@@ -4137,6 +4137,8 @@ void mjCFlex::Compile(const mjVFS* vfs) {
     nvert = (int)vert_.size()/3;
     if (vertbody_.size() == 1) {
       rigid = true;
+    } else if (vertbody_.size() != nvert) {
+      throw mjCError(this, "vertbody size must be 1 or nvert");
     }
   }
   if (nvert < dim+1) {