Feat: add model reroute notification (#12001)

### Summary
Builiding off
https://github.com/openai/codex/pull/11964/files/5c75aa7b89a70bc2cc410a6fd238749306ec4c5e#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/mcp-server/src/codex_tool_runner.rs

@@ -254,6 +254,9 @@ async fn run_codex_tool_session_inner(
                     EventMsg::Warning(_) => {
                         continue;
                     }
+                    EventMsg::ModelReroute(_) => {
+                        continue;
+                    }
                     EventMsg::ElicitationRequest(_) => {
                         // TODO: forward elicitation requests to the client?
                         continue;