Function set_conn_hook 

pub fn set_conn_hook(
    server: &Value,
    hook: Rc<dyn Fn(&Value, &Value) -> Result<(), String>>,
)

Attach an http-style per-connection hook to a server object (called by http::create_server). Stored in the main-thread registry, keyed by the server’s assigned id once it starts listening — so we stash it on the object until listen registers the record.