Function spawn_rpc_proxy_rejecting_method_after 

pub async fn spawn_rpc_proxy_rejecting_method_after(
    endpoint: String,
    method: &'static str,
    successful_calls: usize,
) -> String

Spawns an RPC proxy that rejects method after forwarding successful_calls requests.

Rejections use an HTTP 403 response with a vendor-specific JSON-RPC error code. This models gateways that deny unknown or custom methods without using the standard method-not-found code.