Implement redraw debounce (#1599)

## Summary
- debouce redraw events so repeated requests don't overwhelm the
terminal
- add `RequestRedraw` event and schedule redraws after 100ms

## Testing
- `cargo clippy --tests`
- `cargo test` *(fails: Sandbox Denied errors in landlock tests)*

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https://chatgpt.com/codex/tasks/task_i_68792a65b8b483218ec90a8f68746cd8

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Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/tui/src/app_event.rs

@@ -8,6 +8,10 @@ use crate::slash_command::SlashCommand;
 pub(crate) enum AppEvent {
     CodexEvent(Event),
 
+    /// Request a redraw which will be debounced by the [`App`].
+    RequestRedraw,
+
+    /// Actually draw the next frame.
     Redraw,
 
     KeyEvent(KeyEvent),