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/client.rs

@@ -1,8 +1,6 @@
 use std::sync::Arc;
 use std::time::Duration;
 
-use tokio::io::AsyncRead;
-use tokio::io::AsyncWrite;
 use tokio::time::timeout;
 use tokio_tungstenite::connect_async;
 use tracing::warn;
@@ -136,22 +134,6 @@ impl ExecServerClient {
         Ok(client)
     }
 
-    pub async fn connect_stdio<R, W>(
-        stdin: W,
-        stdout: R,
-        options: ExecServerClientConnectOptions,
-    ) -> Result<Self, ExecServerError>
-    where
-        R: AsyncRead + Unpin + Send + 'static,
-        W: AsyncWrite + Unpin + Send + 'static,
-    {
-        Self::connect(
-            JsonRpcConnection::from_stdio(stdout, stdin, "exec-server stdio".to_string()),
-            options,
-        )
-        .await
-    }
-
     pub async fn connect_websocket(
         args: RemoteExecServerConnectArgs,
     ) -> Result<Self, ExecServerError> {