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/tests/initialize.rs

@@ -0,0 +1,34 @@
+#![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 {});
+
+    server.shutdown().await?;
+    Ok(())
+}