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/core/src/codex.rs

@@ -172,6 +172,8 @@ use crate::protocol::Event;
 use crate::protocol::EventMsg;
 use crate::protocol::ExecApprovalRequestEvent;
 use crate::protocol::McpServerRefreshConfig;
+use crate::protocol::ModelRerouteEvent;
+use crate::protocol::ModelRerouteReason;
 use crate::protocol::NetworkApprovalContext;
 use crate::protocol::Op;
 use crate::protocol::PlanDeltaEvent;
@@ -2440,7 +2442,7 @@ impl Session {
         turn_context: &Arc<TurnContext>,
         server_model: String,
     ) -> bool {
-        let requested_model = turn_context.model_info.slug.as_str();
+        let requested_model = turn_context.model_info.slug.clone();
         if server_model == requested_model {
             info!("server reported model {server_model} (matches requested model)");
             return false;
@@ -2452,6 +2454,16 @@ impl Session {
             "Your account was flagged for potentially high-risk cyber activity and this request was routed to gpt-5.2 as a fallback. To regain access to gpt-5.3-codex, apply for trusted access: {CYBER_VERIFY_URL} or learn more: {CYBER_SAFETY_URL}"
         );
 
+        self.send_event(
+            turn_context,
+            EventMsg::ModelReroute(ModelRerouteEvent {
+                from_model: requested_model.clone(),
+                to_model: server_model.clone(),
+                reason: ModelRerouteReason::HighRiskCyberActivity,
+            }),
+        )
+        .await;
+
         self.send_event(
             turn_context,
             EventMsg::Warning(WarningEvent {