Add code_mode experimental feature (#13418)

A much narrower and more isolated (no node features) version of js_repl

codex-rs/core/tests/suite/mod.rs

@@ -65,6 +65,7 @@ mod auth_refresh;
 mod cli_stream;
 mod client;
 mod client_websockets;
+mod code_mode;
 mod codex_delegate;
 mod collaboration_instructions;
 mod compact;