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/bottom_pane_view.rs

@@ -0,0 +1,56 @@
+use crossterm::event::KeyEvent;
+use ratatui::buffer::Buffer;
+use ratatui::layout::Rect;
+use std::sync::mpsc::SendError;
+
+use crate::app_event::AppEvent;
+use crate::user_approval_widget::ApprovalRequest;
+
+use super::BottomPane;
+
+/// Type to use for a method that may require a redraw of the UI.
+pub(crate) enum ConditionalUpdate {
+    NeedsRedraw,
+    NoRedraw,
+}
+
+/// Trait implemented by every view that can be shown in the bottom pane.
+pub(crate) trait BottomPaneView<'a> {
+    /// Handle a key event while the view is active. A redraw is always
+    /// scheduled after this call.
+    fn handle_key_event(
+        &mut self,
+        pane: &mut BottomPane<'a>,
+        key_event: KeyEvent,
+    ) -> Result<(), SendError<AppEvent>>;
+
+    /// Return `true` if the view has finished and should be removed.
+    fn is_complete(&self) -> bool {
+        false
+    }
+
+    /// Height required to render the view.
+    fn calculate_required_height(&self, area: &Rect) -> u16;
+
+    /// Render the view: this will be displayed in place of the composer.
+    fn render(&self, area: Rect, buf: &mut Buffer);
+
+    /// Update the status indicator text.
+    fn update_status_text(&mut self, _text: String) -> ConditionalUpdate {
+        ConditionalUpdate::NoRedraw
+    }
+
+    /// Called when task completes to check if the view should be hidden.
+    fn should_hide_when_task_is_done(&mut self) -> bool {
+        false
+    }
+
+    /// Try to handle approval request; return the original value if not
+    /// consumed.
+    fn try_consume_approval_request(
+        &mut self,
+        request: ApprovalRequest,
+    ) -> Option<ApprovalRequest> {
+        Some(request)
+    }
+}