Function fetch_send 

pub fn fetch_send(
    method: &str,
    url: &str,
    body: &str,
    headers: Vec<(String, String)>,
    closure: RuntimeValue,
) -> i64

fetchSend(method, url, body, headers, onDone) -> Int — register a request, returning its correlation id. Contract violations (oversized body, disallowed header) fail the request immediately with a structured error.