start method
Implementation
Future<void> start() async {
if (_server != null) {
throw StateError('App is already started');
}
// Create WebhookServer hosts for each ServicePath
_hosts = paths.map(_createHost).toList();
// Start all hosts (they may bind routes / internal state)
await Future.wait(_hosts!.map((h) => h.start()));
// Finally start the HTTP server
await _startServer();
}