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/protocol/src/items.rs

@@ -0,0 +1,33 @@
+use crate::user_input::UserInput;
+use schemars::JsonSchema;
+use serde::Deserialize;
+use serde::Serialize;
+use ts_rs::TS;
+
+#[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema)]
+pub enum TurnItem {
+    UserMessage(UserMessageItem),
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema)]
+pub struct UserMessageItem {
+    pub id: String,
+    pub content: Vec<UserInput>,
+}
+
+impl UserMessageItem {
+    pub fn new(content: &[UserInput]) -> Self {
+        Self {
+            id: uuid::Uuid::new_v4().to_string(),
+            content: content.to_vec(),
+        }
+    }
+}
+
+impl TurnItem {
+    pub fn id(&self) -> String {
+        match self {
+            TurnItem::UserMessage(item) => item.id.clone(),
+        }
+    }
+}