[tui] Show speed in session header (#13446)

- add a speed row to the startup/session header under the model row
- render the speed row with the same styling pattern as the model row,
using /fast to change
- show only Fast or Standard to users and update the affected snapshots

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui/src/app.rs

@@ -806,6 +806,8 @@ impl App {
         history_cell::SessionHeaderHistoryCell::new(
             self.chat_widget.current_model().to_string(),
             self.chat_widget.current_reasoning_effort(),
+            self.chat_widget
+                .should_show_fast_status(self.chat_widget.current_service_tier()),
             self.config.cwd.clone(),
             version,
         )
@@ -5178,6 +5180,7 @@ mod tests {
                 is_first,
                 None,
                 None,
+                false,
             )) as Arc<dyn HistoryCell>
         };
 
@@ -5827,6 +5830,7 @@ mod tests {
                 is_first,
                 None,
                 None,
+                false,
             )) as Arc<dyn HistoryCell>
         };