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()?;