Make model switcher two-stage (#4178)

https://github.com/user-attachments/assets/16d5c67c-e580-4a29-983c-a315f95424ee

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__model_reasoning_selection_popup.snap

@@ -0,0 +1,13 @@
+---
+source: tui/src/chatwidget/tests.rs
+expression: popup
+---
+  Select Reasoning Level
+  Reasoning for model gpt-5-codex
+
+  1. Low               Fastest responses with limited reasoning
+  2. Medium (default)  Dynamically adjusts reasoning based on the task
+› 3. High (current)    Maximizes reasoning depth for complex or ambiguous
+                       problems
+
+  Press enter to confirm or esc to go back

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__model_selection_popup.snap

@@ -0,0 +1,12 @@
+---
+source: tui/src/chatwidget/tests.rs
+expression: popup
+---
+  Select Model
+  Switch the model for this and future Codex CLI sessions
+
+  1. gpt-5                  Broad world knowledge with strong general
+                            reasoning.
+› 2. gpt-5-codex (current)  Optimized for coding tasks with many tools.
+
+  Press enter to confirm or esc to go back

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

@@ -936,6 +936,65 @@ fn render_bottom_first_row(chat: &ChatWidget, width: u16) -> String {
     String::new()
 }
 
+fn render_bottom_popup(chat: &ChatWidget, width: u16) -> String {
+    let height = chat.desired_height(width);
+    let area = Rect::new(0, 0, width, height);
+    let mut buf = Buffer::empty(area);
+    (chat).render_ref(area, &mut buf);
+
+    let mut lines: Vec<String> = (0..area.height)
+        .map(|row| {
+            let mut line = String::new();
+            for col in 0..area.width {
+                let symbol = buf[(area.x + col, area.y + row)].symbol();
+                if symbol.is_empty() {
+                    line.push(' ');
+                } else {
+                    line.push_str(symbol);
+                }
+            }
+            line.trim_end().to_string()
+        })
+        .collect();
+
+    while lines.first().is_some_and(|line| line.trim().is_empty()) {
+        lines.remove(0);
+    }
+    while lines.last().is_some_and(|line| line.trim().is_empty()) {
+        lines.pop();
+    }
+
+    lines.join("\n")
+}
+
+#[test]
+fn model_selection_popup_snapshot() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual();
+
+    chat.config.model = "gpt-5-codex".to_string();
+    chat.open_model_popup();
+
+    let popup = render_bottom_popup(&chat, 80);
+    assert_snapshot!("model_selection_popup", popup);
+}
+
+#[test]
+fn model_reasoning_selection_popup_snapshot() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual();
+
+    chat.config.model = "gpt-5-codex".to_string();
+    chat.config.model_reasoning_effort = Some(ReasoningEffortConfig::High);
+
+    let presets = builtin_model_presets(None)
+        .into_iter()
+        .filter(|preset| preset.model == "gpt-5-codex")
+        .collect::<Vec<_>>();
+    chat.open_reasoning_popup("gpt-5-codex".to_string(), presets);
+
+    let popup = render_bottom_popup(&chat, 80);
+    assert_snapshot!("model_reasoning_selection_popup", popup);
+}
+
 #[test]
 fn exec_history_extends_previous_when_consecutive() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual();