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/suite/items.rs

@@ -0,0 +1,68 @@
+#![cfg(not(target_os = "windows"))]
+
+use anyhow::Ok;
+use codex_core::protocol::EventMsg;
+use codex_core::protocol::Op;
+use codex_protocol::items::TurnItem;
+use codex_protocol::user_input::UserInput;
+use core_test_support::responses;
+use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_response_created;
+use core_test_support::responses::sse;
+use core_test_support::responses::start_mock_server;
+use core_test_support::skip_if_no_network;
+use core_test_support::test_codex::TestCodex;
+use core_test_support::test_codex::test_codex;
+use core_test_support::wait_for_event_match;
+use pretty_assertions::assert_eq;
+use wiremock::matchers::any;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn user_message_item_is_emitted() -> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+
+    let TestCodex { codex, .. } = test_codex().build(&server).await?;
+
+    let first_response = sse(vec![ev_response_created("resp-1"), ev_completed("resp-1")]);
+    responses::mount_sse_once_match(&server, any(), first_response).await;
+
+    codex
+        .submit(Op::UserInput {
+            items: (vec![UserInput::Text {
+                text: "please inspect sample.txt".into(),
+            }]),
+        })
+        .await?;
+
+    let started = wait_for_event_match(&codex, |ev| match ev {
+        EventMsg::ItemStarted(e) => Some(e.clone()),
+        _ => None,
+    })
+    .await;
+
+    let completed = wait_for_event_match(&codex, |ev| match ev {
+        EventMsg::ItemCompleted(e) => Some(e.clone()),
+        _ => None,
+    })
+    .await;
+
+    let TurnItem::UserMessage(started_item) = started.item;
+    let TurnItem::UserMessage(completed_item) = completed.item;
+
+    assert_eq!(started_item.id, completed_item.id);
+    assert_eq!(
+        started_item.content,
+        vec![UserInput::Text {
+            text: "please inspect sample.txt".into(),
+        }]
+    );
+    assert_eq!(
+        completed_item.content,
+        vec![UserInput::Text {
+            text: "please inspect sample.txt".into(),
+        }]
+    );
+    Ok(())
+}