Add codex-exec-server crate

This adds the standalone exec-server stdio JSON-RPC crate and its
smoke tests without wiring it into the CLI or unified-exec yet.

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

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

@@ -0,0 +1,7 @@
+#[tokio::main]
+async fn main() {
+    if let Err(err) = codex_exec_server::run_main().await {
+        eprintln!("{err}");
+        std::process::exit(1);
+    }
+}