app-server: Add back pressure and batching to command/exec (#15547)

* Add
`OutgoingMessageSender::send_server_notification_to_connection_and_wait`
which returns only once message is written to websocket (or failed to do
so)
* Use this mechanism to apply back pressure to stdout/stderr streams of
processes spawned by `command/exec`, to limit them to at most one
message in-memory at a time
* Use back pressure signal to also batch smaller chunks into ≈64KiB ones

This should make commands execution more robust over
high-latency/low-throughput networks

codex-rs/app-server/src/lib.rs

@@ -22,6 +22,7 @@ use crate::message_processor::MessageProcessorArgs;
 use crate::outgoing_message::ConnectionId;
 use crate::outgoing_message::OutgoingEnvelope;
 use crate::outgoing_message::OutgoingMessageSender;
+use crate::outgoing_message::QueuedOutgoingMessage;
 use crate::transport::CHANNEL_CAPACITY;
 use crate::transport::ConnectionState;
 use crate::transport::OutboundConnectionState;
@@ -103,7 +104,7 @@ enum OutboundControlEvent {
     /// Register a new writer for an opened connection.
     Opened {
         connection_id: ConnectionId,
-        writer: mpsc::Sender<crate::outgoing_message::OutgoingMessage>,
+        writer: mpsc::Sender<QueuedOutgoingMessage>,
         disconnect_sender: Option<CancellationToken>,
         initialized: Arc<AtomicBool>,
         experimental_api_enabled: Arc<AtomicBool>,