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/exec/src/event_processor_with_human_output.rs

@@ -188,6 +188,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                     "warning:".style(self.yellow).style(self.bold)
                 );
             }
+            EventMsg::ModelReroute(_) => {}
             EventMsg::DeprecationNotice(DeprecationNoticeEvent { summary, details }) => {
                 ts_msg!(
                     self,