Move process-oriented client helpers and exports into the exec follow-up. Keep the first PR focused on spawning the stub server and running the initialize handshake.\n\nCo-authored-by: Codex <noreply@openai.com>
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>
