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/app_event_sender.rs

@@ -0,0 +1,22 @@
+use std::sync::mpsc::Sender;
+
+use crate::app_event::AppEvent;
+
+#[derive(Clone, Debug)]
+pub(crate) struct AppEventSender {
+    app_event_tx: Sender<AppEvent>,
+}
+
+impl AppEventSender {
+    pub(crate) fn new(app_event_tx: Sender<AppEvent>) -> Self {
+        Self { app_event_tx }
+    }
+
+    /// Send an event to the app event channel. If it fails, we swallow the
+    /// error and log it.
+    pub(crate) fn send(&self, event: AppEvent) {
+        if let Err(e) = self.app_event_tx.send(event) {
+            tracing::error!("failed to send event: {e}");
+        }
+    }
+}