chore: move each view used in BottomPane into its own file (#928)

`BottomPane` was getting a bit unwieldy because it maintained a
`PaneState` enum with three variants and many of its methods had `match`
statements to handle each variant. To replace the enum, this PR:

* Introduces a `trait BottomPaneView` that has two implementations:
`StatusIndicatorView` and `ApprovalModalView`.
* Migrates `PaneState::TextInput` into its own struct, `ChatComposer`,
that does **not** implement `BottomPaneView`.
* Updates `BottomPane` so it has `composer: ChatComposer` and
`active_view: Option<Box<dyn BottomPaneView<'a> + 'a>>`. The idea is
that `active_view` takes priority and is displayed when it is `Some`;
otherwise, `ChatComposer` is displayed.
* While methods of `BottomPane` often have to check whether
`active_view` is present to decide which component to delegate to, the
code is more straightforward than before and introducing new
implementations of `BottomPaneView` should be less painful.

Because we want to retain the `TextArea` owned by `ChatComposer` even
when another view is displayed, to keep the ownership logic simple, it
seemed best to keep `ChatComposer` distinct from `BottomPaneView`.

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

@@ -0,0 +1,57 @@
+use std::sync::mpsc::SendError;
+use std::sync::mpsc::Sender;
+
+use crossterm::event::KeyEvent;
+use ratatui::buffer::Buffer;
+use ratatui::layout::Rect;
+use ratatui::widgets::WidgetRef;
+
+use crate::app_event::AppEvent;
+use crate::status_indicator_widget::StatusIndicatorWidget;
+
+use super::BottomPane;
+use super::BottomPaneView;
+use super::bottom_pane_view::ConditionalUpdate;
+
+pub(crate) struct StatusIndicatorView {
+    view: StatusIndicatorWidget,
+}
+
+impl StatusIndicatorView {
+    pub fn new(app_event_tx: Sender<AppEvent>, height: u16) -> Self {
+        Self {
+            view: StatusIndicatorWidget::new(app_event_tx, height),
+        }
+    }
+
+    pub fn update_text(&mut self, text: String) {
+        self.view.update_text(text);
+    }
+}
+
+impl<'a> BottomPaneView<'a> for StatusIndicatorView {
+    fn handle_key_event(
+        &mut self,
+        _pane: &mut BottomPane<'a>,
+        _key_event: KeyEvent,
+    ) -> Result<(), SendError<AppEvent>> {
+        Ok(())
+    }
+
+    fn update_status_text(&mut self, text: String) -> ConditionalUpdate {
+        self.update_text(text);
+        ConditionalUpdate::NeedsRedraw
+    }
+
+    fn should_hide_when_task_is_done(&mut self) -> bool {
+        true
+    }
+
+    fn calculate_required_height(&self, _area: &Rect) -> u16 {
+        self.view.get_height()
+    }
+
+    fn render(&self, area: Rect, buf: &mut Buffer) {
+        self.view.render_ref(area, buf);
+    }
+}