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/lib.rs

@@ -331,7 +331,13 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
     info!("Sent prompt with event ID: {initial_prompt_task_id}");
 
     // Run the loop until the task is complete.
+    // Track whether a fatal error was reported by the server so we can
+    // exit with a non-zero status for automation-friendly signaling.
+    let mut error_seen = false;
     while let Some(event) = rx.recv().await {
+        if matches!(event.msg, EventMsg::Error(_)) {
+            error_seen = true;
+        }
         let shutdown: CodexStatus = event_processor.process_event(event);
         match shutdown {
             CodexStatus::Running => continue,
@@ -343,6 +349,9 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
             }
         }
     }
+    if error_seen {
+        std::process::exit(1);
+    }
 
     Ok(())
 }