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/chat_composer.rs

@@ -13,9 +13,8 @@ use tui_textarea::Input;
 use tui_textarea::Key;
 use tui_textarea::TextArea;
 
-use std::sync::mpsc::Sender;
-
 use crate::app_event::AppEvent;
+use crate::app_event_sender::AppEventSender;
 
 use super::command_popup::CommandPopup;
 
@@ -33,11 +32,11 @@ pub enum InputResult {
 pub(crate) struct ChatComposer<'a> {
     textarea: TextArea<'a>,
     command_popup: Option<CommandPopup>,
-    app_event_tx: Sender<AppEvent>,
+    app_event_tx: AppEventSender,
 }
 
 impl ChatComposer<'_> {
-    pub fn new(has_input_focus: bool, app_event_tx: Sender<AppEvent>) -> Self {
+    pub fn new(has_input_focus: bool, app_event_tx: AppEventSender) -> Self {
         let mut textarea = TextArea::default();
         textarea.set_placeholder_text("send a message");
         textarea.set_cursor_line_style(ratatui::style::Style::default());
@@ -113,9 +112,7 @@ impl ChatComposer<'_> {
             } => {
                 if let Some(cmd) = popup.selected_command() {
                     // Send command to the app layer.
-                    if let Err(e) = self.app_event_tx.send(AppEvent::DispatchCommand(*cmd)) {
-                        tracing::error!("failed to send DispatchCommand event: {e}");
-                    }
+                    self.app_event_tx.send(AppEvent::DispatchCommand(*cmd));
 
                     // Clear textarea so no residual text remains.
                     self.textarea.select_all();