Function spawn_rpc_proxy_canned_method 

pub async fn spawn_rpc_proxy_canned_method(
    endpoint: String,
    method: &'static str,
    result: Value,
) -> (String, Arc<AtomicUsize>)

Spawns an RPC proxy that answers method with result instead of forwarding it upstream.

All other methods are forwarded. The returned counter tracks how many method calls reached the proxy, which lets tests assert that a request was never sent upstream.