handleRpc() answered a method it does not implement with "Unsupported method: <method>" and no code, so a site probing for an optional method could not tell "not implemented" from "the call failed" and fall back. It now carries code 4200, which EIP-1193 defines for this case; the message is unchanged. The provider already passes any code through to the page. The new test hands the real background's reply to the provider and checks the page sees 4200. Model: opus-5-5