refactor: update roundtrip function to use enqueue_event for event queuing

- Replaced the `append_events` method with `enqueue_event` in the `roundtrip` function to align with recent changes in event handling.
- Adjusted the event queuing to accept a single event, simplifying the code and improving clarity.

Author: jordcodes

tests/integration_postgres.rs

@@ -71,10 +71,10 @@ async fn roundtrip() -> Result<()> {
 
     let mut session = store.session();
     session.store(id, &customer)?;
-    session.append_events(
+    session.enqueue_event(
         id,
         Expected::Any,
-        vec![Event::new("CustomerRegistered", &customer)],
+        Event::new("CustomerRegistered", &customer),
     )?;
     session.save_changes().await?;