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/src/rollout/policy.rs

@@ -71,6 +71,8 @@ pub(crate) fn should_persist_event_msg(ev: &EventMsg) -> bool {
         | EventMsg::PlanUpdate(_)
         | EventMsg::ShutdownComplete
         | EventMsg::ViewImageToolCall(_)
-        | EventMsg::ConversationPath(_) => false,
+        | EventMsg::ConversationPath(_)
+        | EventMsg::ItemStarted(_)
+        | EventMsg::ItemCompleted(_) => false,
     }
 }