fix: exit cleanly when ShutdownComplete is received (#1864)

Previous to this PR, `ShutdownComplete` was not being handled correctly
in `codex exec`, so it always ended up printing the following to stderr:

```
ERROR codex_exec: Error receiving event: InternalAgentDied
```

Because we were not breaking out of the loop for `ShutdownComplete`,
inevitably `codex.next_event()` would get called again and
`rx_event.recv()` would fail and the error would get mapped to
`InternalAgentDied`:


ea7d3f27bd/codex-rs/core/src/codex.rs (L190-L197)

For reference, https://github.com/openai/codex/pull/1647 introduced the
`ShutdownComplete` variant.

codex-rs/exec/src/lib.rs

@@ -216,10 +216,16 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
                     res = codex.next_event() => match res {
                         Ok(event) => {
                             debug!("Received event: {event:?}");
+
+                            let is_shutdown_complete = matches!(event.msg, EventMsg::ShutdownComplete);
                             if let Err(e) = tx.send(event) {
                                 error!("Error sending event: {e:?}");
                                 break;
                             }
+                            if is_shutdown_complete {
+                                info!("Received shutdown event, exiting event loop.");
+                                break;
+                            }
                         },
                         Err(e) => {
                             error!("Error receiving event: {e:?}");