feat: include "reasoning" messages in Rust TUI (#892)

As shown in the screenshot, we now include reasoning messages from the
model in the TUI under the heading "codex reasoning":


![image](https://github.com/user-attachments/assets/d8eb3dc3-2f9f-4e95-847e-d24b421249a8)

To ensure these are visible by default when using `o4-mini`, this also
changes the default value for `summary` (formerly `generate_summary`,
which is deprecated in favor of `summary` according to the docs) from
unset to `"auto"`.

codex-rs/tui/src/chatwidget.rs

@@ -225,6 +225,10 @@ impl ChatWidget<'_> {
                 self.conversation_history.add_agent_message(message);
                 self.request_redraw()?;
             }
+            EventMsg::AgentReasoning { text } => {
+                self.conversation_history.add_agent_reasoning(text);
+                self.request_redraw()?;
+            }
             EventMsg::TaskStarted => {
                 self.bottom_pane.set_task_running(true)?;
                 self.request_redraw()?;

codex-rs/tui/src/conversation_history_widget.rs

@@ -174,6 +174,10 @@ impl ConversationHistoryWidget {
         self.add_to_history(HistoryCell::new_agent_message(message));
     }
 
+    pub fn add_agent_reasoning(&mut self, text: String) {
+        self.add_to_history(HistoryCell::new_agent_reasoning(text));
+    }
+
     pub fn add_background_event(&mut self, message: String) {
         self.add_to_history(HistoryCell::new_background_event(message));
     }

codex-rs/tui/src/history_cell.rs

@@ -41,6 +41,9 @@ pub(crate) enum HistoryCell {
     /// Message from the agent.
     AgentMessage { lines: Vec<Line<'static>> },
 
+    /// Reasoning event from the agent.
+    AgentReasoning { lines: Vec<Line<'static>> },
+
     /// An exec tool call that has not finished yet.
     ActiveExecCommand {
         call_id: String,
@@ -134,6 +137,15 @@ impl HistoryCell {
         HistoryCell::AgentMessage { lines }
     }
 
+    pub(crate) fn new_agent_reasoning(text: String) -> Self {
+        let mut lines: Vec<Line<'static>> = Vec::new();
+        lines.push(Line::from("codex reasoning".magenta().italic()));
+        append_markdown(&text, &mut lines);
+        lines.push(Line::from(""));
+
+        HistoryCell::AgentReasoning { lines }
+    }
+
     pub(crate) fn new_active_exec_command(call_id: String, command: Vec<String>) -> Self {
         let command_escaped = escape_command(&command);
         let start = Instant::now();
@@ -363,6 +375,7 @@ impl HistoryCell {
             HistoryCell::WelcomeMessage { lines, .. }
             | HistoryCell::UserPrompt { lines, .. }
             | HistoryCell::AgentMessage { lines, .. }
+            | HistoryCell::AgentReasoning { lines, .. }
             | HistoryCell::BackgroundEvent { lines, .. }
             | HistoryCell::ErrorEvent { lines, .. }
             | HistoryCell::SessionInfo { lines, .. }