Add todo-list tool support (#4255)

Adds a 1-per-turn todo-list item and item.updated event

```jsonl
{"type":"item.started","item":{"id":"item_6","item_type":"todo_list","items":[{"text":"Record initial two-step plan  now","completed":false},{"text":"Update progress to next step","completed":false}]}}
{"type":"item.updated","item":{"id":"item_6","item_type":"todo_list","items":[{"text":"Record initial two-step plan  now","completed":true},{"text":"Update progress to next step","completed":false}]}}
{"type":"item.completed","item":{"id":"item_6","item_type":"todo_list","items":[{"text":"Record initial two-step plan  now","completed":true},{"text":"Update progress to next step","completed":false}]}}
```

codex-rs/exec/src/exec_events.rs

@@ -10,30 +10,34 @@ pub enum ConversationEvent {
     SessionCreated(SessionCreatedEvent),
     #[serde(rename = "item.started")]
     ItemStarted(ItemStartedEvent),
+    #[serde(rename = "item.updated")]
+    ItemUpdated(ItemUpdatedEvent),
     #[serde(rename = "item.completed")]
     ItemCompleted(ItemCompletedEvent),
     #[serde(rename = "error")]
     Error(ConversationErrorEvent),
 }
 
-/// Payload describing a newly created conversation item.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
 pub struct SessionCreatedEvent {
     pub session_id: String,
 }
 
-/// Payload describing the start of an existing conversation item.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
 pub struct ItemStartedEvent {
     pub item: ConversationItem,
 }
 
-/// Payload describing the completion of an existing conversation item.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
 pub struct ItemCompletedEvent {
     pub item: ConversationItem,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
+pub struct ItemUpdatedEvent {
+    pub item: ConversationItem,
+}
+
 /// Fatal error emitted by the stream.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
 pub struct ConversationErrorEvent {
@@ -58,6 +62,7 @@ pub enum ConversationItemDetails {
     FileChange(FileChangeItem),
     McpToolCall(McpToolCallItem),
     WebSearch(WebSearchItem),
+    TodoList(TodoListItem),
     Error(ErrorItem),
 }
 
@@ -153,3 +158,14 @@ pub struct WebSearchItem {
 pub struct ErrorItem {
     pub message: String,
 }
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
+pub struct TodoItem {
+    pub text: String,
+    pub completed: bool,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
+pub struct TodoListItem {
+    pub items: Vec<TodoItem>,
+}