Fix exec-server in-order request handling and stdio transport

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

codex-rs/exec-server/tests/initialize.rs

@@ -0,0 +1,39 @@
+#![cfg(unix)]
+
+mod common;
+
+use codex_app_server_protocol::JSONRPCMessage;
+use codex_app_server_protocol::JSONRPCResponse;
+use codex_exec_server::InitializeParams;
+use codex_exec_server::InitializeResponse;
+use common::exec_server::exec_server;
+use pretty_assertions::assert_eq;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn exec_server_accepts_initialize() -> anyhow::Result<()> {
+    let mut server = exec_server().await?;
+    let initialize_id = server
+        .send_request(
+            "initialize",
+            serde_json::to_value(InitializeParams {
+                client_name: "exec-server-test".to_string(),
+            })?,
+        )
+        .await?;
+
+    let response = server.next_event().await?;
+    let JSONRPCMessage::Response(JSONRPCResponse { id, result }) = response else {
+        panic!("expected initialize response");
+    };
+    assert_eq!(id, initialize_id);
+    let initialize_response: InitializeResponse = serde_json::from_value(result)?;
+    assert_eq!(
+        initialize_response,
+        InitializeResponse {
+            protocol_version: "exec-server.v0".to_string()
+        }
+    );
+
+    server.shutdown().await?;
+    Ok(())
+}