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/message_processor/tracing_tests.rs

@@ -390,6 +390,7 @@ async fn read_response<T: serde::de::DeserializeOwned>(
         let crate::outgoing_message::OutgoingEnvelope::ToConnection {
             connection_id,
             message,
+            ..
         } = envelope
         else {
             continue;
@@ -420,6 +421,7 @@ async fn read_thread_started_notification(
             crate::outgoing_message::OutgoingEnvelope::ToConnection {
                 connection_id,
                 message,
+                ..
             } => {
                 if connection_id != TEST_CONNECTION_ID {
                     continue;