Add ItemStarted/ItemCompleted events for UserInputItem (#5306)

Adds a new ItemStarted event and delivers UserMessage as the first item
type (more to come).


Renames `InputItem` to `UserInput` considering we're using the `Item`
suffix for actual items.

codex-rs/core/tests/common/lib.rs

@@ -134,6 +134,14 @@ where
     wait_for_event_with_timeout(codex, predicate, Duration::from_secs(1)).await
 }
 
+pub async fn wait_for_event_match<T, F>(codex: &CodexConversation, matcher: F) -> T
+where
+    F: Fn(&codex_core::protocol::EventMsg) -> Option<T>,
+{
+    let ev = wait_for_event(codex, |ev| matcher(ev).is_some()).await;
+    matcher(&ev).unwrap()
+}
+
 pub async fn wait_for_event_with_timeout<F>(
     codex: &CodexConversation,
     mut predicate: F,