Feat: add model reroute notification (#12001)

### Summary
Builiding off
5c75aa7b89 (diff-058ae8f109a8b84b4b79bbfa45f522c2233b9d9e139696044ae374d50b6196e0),
we have created a `model/rerouted` notification that captures the event
so that consumers can render as expected. Keep the `EventMsg::Warning`
path in core so that this does not affect TUI rendering.

`model/rerouted` is meant to be generic to account for future usage
including capacity planning etc.

codex-rs/tui/src/chatwidget.rs

@@ -4021,6 +4021,7 @@ impl ChatWidget {
                 self.on_rate_limit_snapshot(ev.rate_limits);
             }
             EventMsg::Warning(WarningEvent { message }) => self.on_warning(message),
+            EventMsg::ModelReroute(_) => {}
             EventMsg::Error(ErrorEvent {
                 message,
                 codex_error_info,