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/app-server-protocol/src/protocol/common.rs

@@ -795,6 +795,7 @@ server_notification_definitions! {
     ReasoningTextDelta => "item/reasoning/textDelta" (v2::ReasoningTextDeltaNotification),
     /// Deprecated: Use `ContextCompaction` item type instead.
     ContextCompacted => "thread/compacted" (v2::ContextCompactedNotification),
+    ModelRerouted => "model/rerouted" (v2::ModelReroutedNotification),
     DeprecationNotice => "deprecationNotice" (v2::DeprecationNoticeNotification),
     ConfigWarning => "configWarning" (v2::ConfigWarningNotification),
     FuzzyFileSearchSessionUpdated => "fuzzyFileSearch/sessionUpdated" (FuzzyFileSearchSessionUpdatedNotification),

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -30,6 +30,7 @@ use codex_protocol::protocol::AskForApproval as CoreAskForApproval;
 use codex_protocol::protocol::CodexErrorInfo as CoreCodexErrorInfo;
 use codex_protocol::protocol::CreditsSnapshot as CoreCreditsSnapshot;
 use codex_protocol::protocol::ExecCommandStatus as CoreExecCommandStatus;
+use codex_protocol::protocol::ModelRerouteReason as CoreModelRerouteReason;
 use codex_protocol::protocol::NetworkAccess as CoreNetworkAccess;
 use codex_protocol::protocol::PatchApplyStatus as CorePatchApplyStatus;
 use codex_protocol::protocol::RateLimitSnapshot as CoreRateLimitSnapshot;
@@ -229,6 +230,12 @@ v2_enum_from_core!(
     }
 );
 
+v2_enum_from_core!(
+    pub enum ModelRerouteReason from CoreModelRerouteReason {
+        HighRiskCyberActivity
+    }
+);
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
 #[serde(tag = "type", rename_all = "camelCase")]
 #[ts(tag = "type")]
@@ -3305,6 +3312,17 @@ pub struct AccountLoginCompletedNotification {
     pub error: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ModelReroutedNotification {
+    pub thread_id: String,
+    pub turn_id: String,
+    pub from_model: String,
+    pub to_model: String,
+    pub reason: ModelRerouteReason,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]