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/codex_message_processor.rs

@@ -8871,6 +8871,7 @@ mod tests {
                     request_id: sent_request_id,
                     ..
                 }),
+            ..
         } = request_message
         else {
             panic!("expected tool request to be sent to the subscribed connection");