Dispatch a single parsed message. Returns the JSON-RPC response for
requests, or Nullable.init for notifications (which get no reply).
ctx is the channel for any server->client traffic the handler emits;
when omitted, a NullContext is used (no streaming).
Dispatch a single parsed message. Returns the JSON-RPC response for requests, or Nullable.init for notifications (which get no reply). ctx is the channel for any server->client traffic the handler emits; when omitted, a NullContext is used (no streaming).