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/app-server-protocol/schema/json/v2/ModelReroutedNotification.json

@@ -0,0 +1,37 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "ModelRerouteReason": {
+      "enum": [
+        "highRiskCyberActivity"
+      ],
+      "type": "string"
+    }
+  },
+  "properties": {
+    "fromModel": {
+      "type": "string"
+    },
+    "reason": {
+      "$ref": "#/definitions/ModelRerouteReason"
+    },
+    "threadId": {
+      "type": "string"
+    },
+    "toModel": {
+      "type": "string"
+    },
+    "turnId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "fromModel",
+    "reason",
+    "threadId",
+    "toModel",
+    "turnId"
+  ],
+  "title": "ModelReroutedNotification",
+  "type": "object"
+}