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/schema/json/v1/ResumeConversationResponse.json

@@ -473,6 +473,35 @@
           "title": "WarningEventMsg",
           "type": "object"
         },
+        {
+          "description": "Model routing changed from the requested model to a different model.",
+          "properties": {
+            "from_model": {
+              "type": "string"
+            },
+            "reason": {
+              "$ref": "#/definitions/ModelRerouteReason"
+            },
+            "to_model": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "model_reroute"
+              ],
+              "title": "ModelRerouteEventMsgType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "from_model",
+            "reason",
+            "to_model",
+            "type"
+          ],
+          "title": "ModelRerouteEventMsg",
+          "type": "object"
+        },
         {
           "description": "Conversation history was compacted (either automatically or manually).",
           "properties": {
@@ -3318,6 +3347,12 @@
       ],
       "type": "string"
     },
+    "ModelRerouteReason": {
+      "enum": [
+        "high_risk_cyber_activity"
+      ],
+      "type": "string"
+    },
     "NetworkAccess": {
       "description": "Represents whether outbound network access is available to the agent.",
       "enum": [