Fix #19219 by moving observer triggers out of resource_scope (#19221)

# Objective

Fixes #19219 

## Solution

Instead of calling `world.commands().trigger` and
`world.commands().trigger_targets` whenever each scene is spawned, save
the `instance_id` and optional parent entity to perform all such calls
at the end. This prevents the potential flush of the world command queue
that can happen if `add_child` is called from causing the crash.

## Testing

- Did you test these changes? If so, how?
- Verified that I can no longer reproduce the bug with the instructions
at #19219.
  - Ran `bevy_scene` tests
- Visually verified that the following examples still run as expected
`many_foxes`, `scene` . (should I test any more?)
- Are there any parts that need more testing?
- Pending to run `cargo test` at the root to test that all examples
still build; I will update the PR when that's done
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
  - Run bevy as usual
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?
  - N/a (tested on Linux/wayland but it shouldn't be relevant)

---

crates/bevy_scene/src/scene_spawner.rs

@@ -79,6 +79,7 @@ pub struct SceneSpawner {
     scenes_to_despawn: Vec<AssetId<DynamicScene>>,
     instances_to_despawn: Vec<InstanceId>,
     scenes_with_parent: Vec<(InstanceId, Entity)>,
+    instances_ready: Vec<(InstanceId, Option<Entity>)>,
 }
 
 /// Errors that can occur when spawning a scene.
@@ -337,8 +338,9 @@ impl SceneSpawner {
                     // Scenes with parents need more setup before they are ready.
                     // See `set_scene_instance_parent_sync()`.
                     if parent.is_none() {
-                        // Defer via commands otherwise SceneSpawner is not available in the observer.
+                        // We trigger `SceneInstanceReady` events after processing all scenes
-                        world.commands().trigger(SceneInstanceReady { instance_id });
+                        // SceneSpawner may not be available in the observer.
+                        self.instances_ready.push((instance_id, None));
                     }
                 }
                 Err(SceneSpawnError::NonExistentScene { .. }) => {
@@ -362,8 +364,9 @@ impl SceneSpawner {
                     // Scenes with parents need more setup before they are ready.
                     // See `set_scene_instance_parent_sync()`.
                     if parent.is_none() {
-                        // Defer via commands otherwise SceneSpawner is not available in the observer.
+                        // We trigger `SceneInstanceReady` events after processing all scenes
-                        world.commands().trigger(SceneInstanceReady { instance_id });
+                        // SceneSpawner may not be available in the observer.
+                        self.instances_ready.push((instance_id, None));
                     }
                 }
                 Err(SceneSpawnError::NonExistentRealScene { .. }) => {
@@ -398,12 +401,25 @@ impl SceneSpawner {
                     }
                 }
 
+                // We trigger `SceneInstanceReady` events after processing all scenes
+                // SceneSpawner may not be available in the observer.
+                self.instances_ready.push((instance_id, Some(parent)));
+            } else {
+                self.scenes_with_parent.push((instance_id, parent));
+            }
+        }
+    }
+
+    fn trigger_scene_ready_events(&mut self, world: &mut World) {
+        for (instance_id, parent) in self.instances_ready.drain(..) {
+            if let Some(parent) = parent {
                 // Defer via commands otherwise SceneSpawner is not available in the observer.
                 world
                     .commands()
                     .trigger_targets(SceneInstanceReady { instance_id }, parent);
             } else {
-                self.scenes_with_parent.push((instance_id, parent));
+                // Defer via commands otherwise SceneSpawner is not available in the observer.
+                world.commands().trigger(SceneInstanceReady { instance_id });
             }
         }
     }
@@ -477,6 +493,7 @@ pub fn scene_spawner_system(world: &mut World) {
             .update_spawned_scenes(world, &updated_spawned_scenes)
             .unwrap();
         scene_spawner.set_scene_instance_parent_sync(world);
+        scene_spawner.trigger_scene_ready_events(world);
     });
 }