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/state/item_collector.rs

@@ -0,0 +1,68 @@
+use async_channel::Sender;
+use codex_protocol::ConversationId;
+use codex_protocol::items::TurnItem;
+use codex_protocol::protocol::Event;
+use codex_protocol::protocol::EventMsg;
+use codex_protocol::protocol::ItemCompletedEvent;
+use codex_protocol::protocol::ItemStartedEvent;
+use tracing::error;
+
+#[derive(Debug)]
+pub(crate) struct ItemCollector {
+    thread_id: ConversationId,
+    turn_id: String,
+    tx_event: Sender<Event>,
+}
+
+impl ItemCollector {
+    pub fn new(
+        tx_event: Sender<Event>,
+        thread_id: ConversationId,
+        turn_id: String,
+    ) -> ItemCollector {
+        ItemCollector {
+            tx_event,
+            thread_id,
+            turn_id,
+        }
+    }
+
+    pub async fn started(&self, item: TurnItem) {
+        let err = self
+            .tx_event
+            .send(Event {
+                id: self.turn_id.clone(),
+                msg: EventMsg::ItemStarted(ItemStartedEvent {
+                    thread_id: self.thread_id,
+                    turn_id: self.turn_id.clone(),
+                    item,
+                }),
+            })
+            .await;
+        if let Err(e) = err {
+            error!("failed to send item started event: {e}");
+        }
+    }
+
+    pub async fn completed(&self, item: TurnItem) {
+        let err = self
+            .tx_event
+            .send(Event {
+                id: self.turn_id.clone(),
+                msg: EventMsg::ItemCompleted(ItemCompletedEvent {
+                    thread_id: self.thread_id,
+                    turn_id: self.turn_id.clone(),
+                    item,
+                }),
+            })
+            .await;
+        if let Err(e) = err {
+            error!("failed to send item completed event: {e}");
+        }
+    }
+
+    pub async fn started_completed(&self, item: TurnItem) {
+        self.started(item.clone()).await;
+        self.completed(item).await;
+    }
+}

codex-rs/core/src/state/mod.rs

@@ -1,7 +1,9 @@
+mod item_collector;
 mod service;
 mod session;
 mod turn;
 
+pub(crate) use item_collector::ItemCollector;
 pub(crate) use service::SessionServices;
 pub(crate) use session::SessionState;
 pub(crate) use turn::ActiveTurn;