If a JSON-RPC request is received but no matching response is ever sent (e.g., the handler throws asynchronously, the client disconnects mid-request, or a notification handler path completes without a response), the request ID remains in `pendingRequestIds` forever and `onRequestCompleted` is never called to balance the earlier `onRequestStarted`. Because this module powers the new idle-timeout shutdown, a single orphaned request will keep the active counter above zero and prevent the server from ever shutting down on idle, defeating the feature. The synchronous try/catch around `downstreamOnMessage` only handles thrown (sync) errors; rejected promises from an async downstream handler will not trigger the cleanup path.