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/chatwidget.rs

@@ -431,7 +431,7 @@ impl ChatWidget<'_> {
     }
 
     fn request_redraw(&mut self) {
-        self.app_event_tx.send(AppEvent::Redraw);
+        self.app_event_tx.send(AppEvent::RequestRedraw);
     }
 
     pub(crate) fn add_diff_output(&mut self, diff_output: String) {