chore: warn when Code Mode lacks model metadata (#29490)

codex-rs/tui/src/chatwidget/tests/snapshots/codex_tui__chatwidget__tests__status_and_layout__unsupported_code_mode_warning.snap

@@ -0,0 +1,8 @@
+---
+source: tui/src/chatwidget/tests/status_and_layout.rs
+expression: "lines_to_single_string(&cells[0])"
+---
+⚠ Code Mode is enabled in configuration, but model `gpt-5.4` does not advertise
+  Code Mode support. This may degrade model performance. Disable
+  `features.code_mode` and `features.code_mode_only`, or select a model whose
+  metadata enables Code Mode.

codex-rs/tui/src/chatwidget/tests/status_and_layout.rs

@@ -2079,6 +2079,22 @@ async fn warning_event_adds_warning_history_cell() {
     );
 }
 
+#[tokio::test]
+async fn unsupported_code_mode_warning_renders_as_warning_history_cell() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    handle_warning(
+        &mut chat,
+        "Code Mode is enabled in configuration, but model `gpt-5.4` does not advertise Code Mode support. This may degrade model performance. Disable `features.code_mode` and `features.code_mode_only`, or select a model whose metadata enables Code Mode.",
+    );
+
+    let cells = drain_insert_history(&mut rx);
+    assert_eq!(cells.len(), 1, "expected one warning history cell");
+    insta::assert_snapshot!(
+        "unsupported_code_mode_warning",
+        lines_to_single_string(&cells[0])
+    );
+}
+
 #[tokio::test]
 async fn repeated_model_metadata_warning_is_hidden_for_same_slug() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;