Request admission for the native Streamable HTTP listener.

A gate sees the parsed request method, path, and headers before the body is read. It can serve a separate route, refuse a request with an HTTP response, or return trusted request identity for Snodo.Authorization. The listener does not interpret credentials. A gate that raises or returns an invalid result fails closed with HTTP 500.

Configure {module, options} as the listener's :request_gate option. The module is initialized once at listener startup with the listener's endpoint path in %{path: path}. check/2 runs in each connection's admission task and is bounded by :request_gate_timeout. External calls should also have their own bounds and cleanup on timeout.