Modify SimulationState proto to correspond to mj_getState.

PiperOrigin-RevId: 823494487
Change-Id: Ib284e3165ca691f5b690a376f7b28e51122e1bc4

src/experimental/toolbox/physics.cc

@@ -49,9 +49,9 @@ void Physics::LoadModel(std::string model_file, const mjVFS* vfs) {
   vfs_ = vfs;
 }
 
-void Physics::ProcessPendingLoad() {
+bool Physics::ProcessPendingLoad() {
   if (!pending_load_.has_value()) {
-    return;
+    return model_ && data_;
   }
 
   Clear();
@@ -68,6 +68,8 @@ void Physics::ProcessPendingLoad() {
   on_model_loaded_(model_file);
 
   InitHistory();
+
+  return model_ && data_;
 }
 
 void Physics::Clear() {

src/experimental/toolbox/physics.h

@@ -93,8 +93,9 @@ class Physics {
   // Returns the error message from the simulation.
   std::string_view GetError() const { return error_; }
 
+  bool ProcessPendingLoad();
+
  private:
-  void ProcessPendingLoad();
   void InitHistory();
   void AddToHistory();