Remove stdio transport from exec server (#15119)

Summary
- delete the deprecated stdio transport plumbing from the exec server
stack
- add a basic `exec_server()` harness plus test utilities to start a
server, send requests, and await events
- refresh exec-server dependencies, configs, and documentation to
reflect the new flow

Testing
- Not run (not requested)

---------

Co-authored-by: starr-openai <starr@openai.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/src/connection.rs

@@ -1,16 +1,21 @@
 use codex_app_server_protocol::JSONRPCMessage;
 use futures::SinkExt;
 use futures::StreamExt;
-use tokio::io::AsyncBufReadExt;
 use tokio::io::AsyncRead;
 use tokio::io::AsyncWrite;
-use tokio::io::AsyncWriteExt;
-use tokio::io::BufReader;
-use tokio::io::BufWriter;
 use tokio::sync::mpsc;
 use tokio_tungstenite::WebSocketStream;
 use tokio_tungstenite::tungstenite::Message;
 
+#[cfg(test)]
+use tokio::io::AsyncBufReadExt;
+#[cfg(test)]
+use tokio::io::AsyncWriteExt;
+#[cfg(test)]
+use tokio::io::BufReader;
+#[cfg(test)]
+use tokio::io::BufWriter;
+
 pub(crate) const CHANNEL_CAPACITY: usize = 128;
 
 #[derive(Debug)]
@@ -27,6 +32,7 @@ pub(crate) struct JsonRpcConnection {
 }
 
 impl JsonRpcConnection {
+    #[cfg(test)]
     pub(crate) fn from_stdio<R, W>(reader: R, writer: W, connection_label: String) -> Self
     where
         R: AsyncRead + Unpin + Send + 'static,
@@ -256,6 +262,7 @@ async fn send_malformed_message(
         .await;
 }
 
+#[cfg(test)]
 async fn write_jsonrpc_line_message<W>(
     writer: &mut BufWriter<W>,
     message: &JSONRPCMessage,