Add gRPC TCP transport to the code-mode host (#37745)

## What changed

- Accept `grpc://IP:PORT` endpoints through `--listen` and serve the existing
  code-mode gRPC service over TCP.
- Print the bound HTTP endpoint to stdout so callers can discover the port when
  binding to port `0`.
- Apply the protocol frame-size limits and disable Nagle's algorithm on accepted
  connections.

## Testing

- Add an end-to-end test that starts the host on an ephemeral port, connects a
  gRPC client, and opens a session.
- Verify accepted gRPC sockets have `TCP_NODELAY` enabled.

GitOrigin-RevId: 51d6c21dff8cffb47068c0677ba10ff370385cec

codex-rs/code-mode-host/src/grpc_transport.rs

@@ -0,0 +1,43 @@
+use std::io;
+use std::io::Write as _;
+use std::net::SocketAddr;
+
+use anyhow::Context;
+use anyhow::Result;
+use codex_code_mode_protocol::grpc::code_mode_host_server::CodeModeHostServer;
+use codex_code_mode_protocol::host::MAX_FRAME_BYTES;
+use tokio::net::TcpListener;
+use tonic::transport::Server;
+use tonic::transport::server::TcpIncoming;
+use tracing::info;
+
+use crate::GrpcCodeModeHost;
+
+pub(super) async fn run_tcp_listener(bind_address: SocketAddr) -> Result<()> {
+    let listener = bind_tcp_listener(bind_address).await?;
+    let local_address = listener
+        .local_addr()
+        .context("failed to read code-mode gRPC listen address")?;
+    info!("codex-code-mode-host listening on http://{local_address}");
+    println!("http://{local_address}");
+    io::stdout()
+        .flush()
+        .context("failed to publish code-mode gRPC listen address")?;
+
+    Server::builder()
+        .add_service(
+            CodeModeHostServer::new(GrpcCodeModeHost::new())
+                .max_decoding_message_size(MAX_FRAME_BYTES)
+                .max_encoding_message_size(MAX_FRAME_BYTES),
+        )
+        .serve_with_incoming(listener)
+        .await
+        .context("code-mode gRPC TCP listener failed")
+}
+
+pub(super) async fn bind_tcp_listener(bind_address: SocketAddr) -> Result<TcpIncoming> {
+    let listener = TcpListener::bind(bind_address)
+        .await
+        .with_context(|| format!("failed to bind code-mode gRPC host to {bind_address}"))?;
+    Ok(TcpIncoming::from(listener).with_nodelay(Some(true)))
+}