chore: introduce AppEventSender to help fix clippy warnings and update to Rust 1.87 (#948)

Moving to Rust 1.87 introduced a clippy warning that
`SendError<AppEvent>` was too large.

In practice, the only thing we ever did when we got this error was log
it (if the mspc channel is closed, then the app is likely shutting down
or something, so there's not much to do...), so this finally motivated
me to introduce `AppEventSender`, which wraps
`std::sync::mpsc::Sender<AppEvent>` with a `send()` method that invokes
`send()` on the underlying `Sender` and logs an `Err` if it gets one.

This greatly simplifies the code, as many functions that previously
returned `Result<(), SendError<AppEvent>>` now return `()`, so we don't
have to propagate an `Err` all over the place that we don't really
handle, anyway.

This also makes it so we can upgrade to Rust 1.87 in CI.

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

@@ -1,10 +1,7 @@
+use crate::user_approval_widget::ApprovalRequest;
 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;
 
@@ -18,11 +15,7 @@ pub(crate) enum ConditionalUpdate {
 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>>;
+    fn handle_key_event(&mut self, _pane: &mut BottomPane<'a>, _key_event: KeyEvent) {}
 
     /// Return `true` if the view has finished and should be removed.
     fn is_complete(&self) -> bool {