Add generic RPC server glue to exec-server stub

Introduce API-agnostic server envelope parsing/encoding and a tiny method-registration router in the initialize-only exec-server slice.

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

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

@@ -1,5 +1,6 @@
 mod client;
 mod protocol;
+mod rpc;
 mod server;
 mod server_process;