Increase app-server WebSocket outbound buffer (#19246)

Fixes #18203.

## Why

Remote TUI clients connected through `codex app-server --listen
ws://...` can receive short bursts of outbound turn and tool-output
notifications. The WebSocket transport previously used the shared
128-message channel capacity for its outbound writer queue, so a healthy
client that briefly lagged during normal output streaming could fill the
queue and be disconnected immediately.

This is a smaller mitigation than #18265: instead of adding a new
overflow/backpressure pipeline, keep the existing non-blocking router
behavior and give WebSocket clients enough bounded headroom for
realistic bursts.

## What Changed

- Added a WebSocket-only outbound writer capacity of `64 * 1024`
messages.
- Used that larger capacity only for the WebSocket data writer queue in
`codex-rs/app-server/src/transport/websocket.rs`.
- Left the shared `CHANNEL_CAPACITY` and the existing disconnect-on-full
behavior unchanged for internal/control channels and genuinely stuck
clients.

## Verification

- `cargo test -p codex-app-server
transport::tests::broadcast_does_not_block_on_slow_connection`
- Manually retried the #18203 repro prompt against the remote TUI and
confirmed it stayed connected.

codex-rs/app-server/src/transport/websocket.rs

@@ -43,6 +43,11 @@ use tracing::error;
 use tracing::info;
 use tracing::warn;
 
+/// WebSocket clients can briefly lag behind normal turn output bursts while the
+/// writer task is healthy, so give them more headroom than internal channels.
+const WEBSOCKET_OUTBOUND_CHANNEL_CAPACITY: usize = 32 * 1024;
+const _: () = assert!(WEBSOCKET_OUTBOUND_CHANNEL_CAPACITY > CHANNEL_CAPACITY);
+
 fn colorize(text: &str, style: Style) -> String {
     text.if_supports_color(Stream::Stderr, |value| value.style(style))
         .to_string()
@@ -174,7 +179,8 @@ pub(crate) async fn run_websocket_connection<M, SinkError, StreamError>(
     StreamError: std::fmt::Display + Send + 'static,
 {
     let connection_id = next_connection_id();
-    let (writer_tx, writer_rx) = mpsc::channel::<QueuedOutgoingMessage>(CHANNEL_CAPACITY);
+    let (writer_tx, writer_rx) =
+        mpsc::channel::<QueuedOutgoingMessage>(WEBSOCKET_OUTBOUND_CHANNEL_CAPACITY);
     let writer_tx_for_reader = writer_tx.clone();
     let disconnect_token = CancellationToken::new();
     if transport_event_tx