Feat: add upgrade to app server modelList (#10556)

### Summary
* Add model upgrade to listModel app server endpoint to support
dynamically show model upgrade banner.

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

@@ -998,6 +998,7 @@ pub struct ModelListParams {
 pub struct Model {
     pub id: String,
     pub model: String,
+    pub upgrade: Option<String>,
     pub display_name: String,
     pub description: String,
     pub supported_reasoning_efforts: Vec<ReasoningEffortOption>,