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);

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

@@ -1473,7 +1473,7 @@ mod tests {
             .timestamp() as u64;
         let mut rng = rand::rngs::StdRng::seed_from_u64(pst_today_seed);
 
-        for _case in 0..10_000 {
+        for _case in 0..500 {
             let mut ta = TextArea::new();
             let mut state = TextAreaState::default();
             // Track element payloads we insert. Payloads use characters '[' and ']' which
@@ -1497,7 +1497,7 @@ mod tests {
             let mut width: u16 = rng.random_range(1..=12);
             let mut height: u16 = rng.random_range(1..=4);
 
-            for _step in 0..200 {
+            for _step in 0..60 {
                 // Mostly stable width/height, occasionally change
                 if rng.random_bool(0.1) {
                     width = rng.random_range(1..=12);