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/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));
     }