improve performance of 'cargo test -p codex-tui' (#2593)

before:

```
$ time cargo test -p codex-tui -q
[...]
cargo test -p codex-tui -q  39.89s user 10.77s system 98% cpu 51.328 total
```

after:

```
$ time cargo test -p codex-tui -q
[...]
cargo test -p codex-tui -q  1.37s user 0.64s system 29% cpu 6.699 total
```

the major offenders were the textarea fuzz test and the custom_terminal
doctests. (i think the doctests were being recompiled every time which
made them extra slow?)

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

@@ -492,8 +492,6 @@ mod tests {
         assert!(pane.active_view.is_some(), "active view should be present");
 
         // Render and ensure the top row includes the Working header instead of the composer.
-        // Give the animation thread a moment to tick.
-        std::thread::sleep(std::time::Duration::from_millis(120));
         let area = Rect::new(0, 0, 40, 3);
         let mut buf = Buffer::empty(area);
         (&pane).render_ref(area, &mut buf);
@@ -525,9 +523,6 @@ mod tests {
         // Begin a task: show initial status.
         pane.set_task_running(true);
 
-        // Allow some frames so the animation thread ticks.
-        std::thread::sleep(std::time::Duration::from_millis(120));
-
         // Render and confirm the line contains the "Working" header.
         let area = Rect::new(0, 0, 40, 3);
         let mut buf = Buffer::empty(area);