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/protocol/src/protocol.rs

@@ -883,6 +883,9 @@ pub enum EventMsg {
     /// indicates the turn continued but the user should still be notified.
     Warning(WarningEvent),
 
+    /// Model routing changed from the requested model to a different model.
+    ModelReroute(ModelRerouteEvent),
+
     /// Conversation history was compacted (either automatically or manually).
     ContextCompacted(ContextCompactedEvent),
 
@@ -1342,6 +1345,20 @@ pub struct WarningEvent {
     pub message: String,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "snake_case")]
+#[ts(rename_all = "snake_case")]
+pub enum ModelRerouteReason {
+    HighRiskCyberActivity,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+pub struct ModelRerouteEvent {
+    pub from_model: String,
+    pub to_model: String,
+    pub reason: ModelRerouteReason,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct ContextCompactedEvent;