Add session header to chat widget (#3592)

<img width="570" height="332" alt="image"
src="https://github.com/user-attachments/assets/ca6dfcb0-f3a1-4b3e-978d-4f844ba77527"
/>

codex-rs/tui/src/app.rs

@@ -295,7 +295,7 @@ impl App {
                 self.on_update_reasoning_effort(effort);
             }
             AppEvent::UpdateModel(model) => {
-                self.chat_widget.set_model(model.clone());
+                self.chat_widget.set_model(&model);
                 self.config.model = model.clone();
                 if let Some(family) = find_family_for_model(&model) {
                     self.config.model_family = family;