Function error_invocation 

pub fn error_invocation(call_id: &str, err: JmapError) -> Invocation

Wrap a method-level error as an error Invocation for methodResponses.

Per RFC 8620 §3.6.2, error invocations always use "error" as the method name regardless of the original method. Only call_id is echoed. Method-level errors are returned inside methodResponses with HTTP 200 — they are NOT returned as top-level HTTP errors.