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/lib.rs

@@ -0,0 +1,20 @@
+mod client;
+mod protocol;
+mod server;
+
+pub use client::ExecServerClient;
+pub use client::ExecServerError;
+pub use client::ExecServerLaunchCommand;
+pub use client::ExecServerProcess;
+pub use protocol::ExecExitedNotification;
+pub use protocol::ExecOutputDeltaNotification;
+pub use protocol::ExecOutputStream;
+pub use protocol::ExecParams;
+pub use protocol::ExecResponse;
+pub use protocol::InitializeParams;
+pub use protocol::InitializeResponse;
+pub use protocol::TerminateParams;
+pub use protocol::TerminateResponse;
+pub use protocol::WriteParams;
+pub use protocol::WriteResponse;
+pub use server::run_main;