Add turn started/completed events and correct exit code on error (#4309)

Adds new event for session completed that includes usage. Also ensures
we return 1 on failures.
```
{
  "type": "session.created",
  "session_id": "019987a7-93e7-7b20-9e05-e90060e411ea"
}
{
  "type": "turn.started"
}
...
{
  "type": "turn.completed",
  "usage": {
    "input_tokens": 78913,
    "cached_input_tokens": 65280,
    "output_tokens": 1099
  }
}
```

codex-rs/exec/src/exec_events.rs

@@ -8,6 +8,10 @@ use ts_rs::TS;
 pub enum ConversationEvent {
     #[serde(rename = "session.created")]
     SessionCreated(SessionCreatedEvent),
+    #[serde(rename = "turn.started")]
+    TurnStarted(TurnStartedEvent),
+    #[serde(rename = "turn.completed")]
+    TurnCompleted(TurnCompletedEvent),
     #[serde(rename = "item.started")]
     ItemStarted(ItemStartedEvent),
     #[serde(rename = "item.updated")]
@@ -23,6 +27,22 @@ pub struct SessionCreatedEvent {
     pub session_id: String,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS, Default)]
+pub struct TurnStartedEvent {}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
+pub struct TurnCompletedEvent {
+    pub usage: Usage,
+}
+
+/// Minimal usage summary for a turn.
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS, Default)]
+pub struct Usage {
+    pub input_tokens: u64,
+    pub cached_input_tokens: u64,
+    pub output_tokens: u64,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, TS)]
 pub struct ItemStartedEvent {
     pub item: ConversationItem,