Add Fast mode status-line indicator (#13670)

Addresses feature request #13660

Adds new option to `/statusline` so the status line can display "fast
on" or "fast off"

Summary
- introduce a `FastMode` status-line item so `/statusline` can render
explicit `Fast on`/`Fast off` text for the service tier
- wire the item into the picker metadata and resolve its string from
`ChatWidget` without adding any unrelated `thread-name` logic or storage
changes
- ensure the refresh paths keep the cached footer in sync when the
service tier (fast mode) changes

Testing
- Manually tested

Here's what it looks like when enabled:

<img width="366" height="75" alt="image"
src="https://github.com/user-attachments/assets/7f992d2b-6dab-49ed-aa43-ad496f56f193"
/>

codex-rs/tui/src/bottom_pane/mod.rs

@@ -690,6 +690,11 @@ impl BottomPane {
         self.status.is_some()
     }
 
+    #[cfg(test)]
+    pub(crate) fn status_line_text(&self) -> Option<String> {
+        self.composer.status_line_text()
+    }
+
     pub(crate) fn show_esc_backtrack_hint(&mut self) {
         self.esc_backtrack_hint = true;
         self.composer.set_esc_backtrack_hint(true);