Update EntityCommands::trigger to check for the entity's existence (#18071)

## Objective

`EntityCommands::trigger` internally uses `Commands::trigger_targets`,
which means it gets queued using `Commands::queue` rather
`EntityCommands::queue`. This previously wouldn't have made much
difference, but now entity commands check whether the entity exists, and
that check never happens in this case.

## Solution

- Add `entity_command::trigger`, which calls the same function as before
(`World::trigger_targets_with_caller`) but through the `EntityWorldMut`
passed to entity commands.
- Change `EntityCommands::trigger` to queue the new entity command
normally.

crates/bevy_ecs/src/system/commands/entity_command.rs

@@ -268,6 +268,19 @@ pub fn observe<E: Event, B: Bundle, M>(
     }
 }
 
+/// An [`EntityCommand`] that sends a [`Trigger`](crate::observer::Trigger) targeting an entity.
+/// This will run any [`Observer`](crate::observer::Observer) of the given [`Event`] watching the entity.
+#[track_caller]
+pub fn trigger(event: impl Event) -> impl EntityCommand {
+    let caller = MaybeLocation::caller();
+    move |mut entity: EntityWorldMut| {
+        let id = entity.id();
+        entity.world_scope(|world| {
+            world.trigger_targets_with_caller(event, id, caller);
+        });
+    }
+}
+
 /// An [`EntityCommand`] that clones parts of an entity onto another entity,
 /// configured through [`EntityClonerBuilder`].
 pub fn clone_with(

crates/bevy_ecs/src/system/commands/mod.rs

@@ -1916,13 +1916,11 @@ impl<'a> EntityCommands<'a> {
         &mut self.commands
     }
 
-    /// Sends a [`Trigger`] targeting this entity. This will run any [`Observer`] of the `event` that
-    /// watches this entity.
-    ///
-    /// [`Trigger`]: crate::observer::Trigger
+    /// Sends a [`Trigger`](crate::observer::Trigger) targeting the entity.
+    /// This will run any [`Observer`] of the given [`Event`] watching this entity.
+    #[track_caller]
     pub fn trigger(&mut self, event: impl Event) -> &mut Self {
-        self.commands.trigger_targets(event, self.entity);
-        self
+        self.queue(entity_command::trigger(event))
     }
 
     /// Creates an [`Observer`] listening for events of type `E` targeting this entity.