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/approval_modal_view.rs

@@ -0,0 +1,73 @@
+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::user_approval_widget::ApprovalRequest;
+use crate::user_approval_widget::UserApprovalWidget;
+
+use super::BottomPane;
+use super::BottomPaneView;
+
+/// Modal overlay asking the user to approve/deny a sequence of requests.
+pub(crate) struct ApprovalModalView<'a> {
+    current: UserApprovalWidget<'a>,
+    queue: Vec<ApprovalRequest>,
+    app_event_tx: Sender<AppEvent>,
+}
+
+impl ApprovalModalView<'_> {
+    pub fn new(request: ApprovalRequest, app_event_tx: Sender<AppEvent>) -> Self {
+        Self {
+            current: UserApprovalWidget::new(request, app_event_tx.clone()),
+            queue: Vec::new(),
+            app_event_tx,
+        }
+    }
+
+    pub fn enqueue_request(&mut self, req: ApprovalRequest) {
+        self.queue.push(req);
+    }
+
+    /// Advance to next request if the current one is finished.
+    fn maybe_advance(&mut self) {
+        if self.current.is_complete() {
+            if let Some(req) = self.queue.pop() {
+                self.current = UserApprovalWidget::new(req, self.app_event_tx.clone());
+            }
+        }
+    }
+}
+
+impl<'a> BottomPaneView<'a> for ApprovalModalView<'a> {
+    fn handle_key_event(
+        &mut self,
+        _pane: &mut BottomPane<'a>,
+        key_event: KeyEvent,
+    ) -> Result<(), SendError<AppEvent>> {
+        self.current.handle_key_event(key_event)?;
+        self.maybe_advance();
+        Ok(())
+    }
+
+    fn is_complete(&self) -> bool {
+        self.current.is_complete() && self.queue.is_empty()
+    }
+
+    fn calculate_required_height(&self, area: &Rect) -> u16 {
+        self.current.get_height(area)
+    }
+
+    fn render(&self, area: Rect, buf: &mut Buffer) {
+        (&self.current).render_ref(area, buf);
+    }
+
+    fn try_consume_approval_request(&mut self, req: ApprovalRequest) -> Option<ApprovalRequest> {
+        self.enqueue_request(req);
+        None
+    }
+}