fix(tcp): cancel pending connects on disconnect and propagate remote close

lib/connector/meshcore_connector_tcp.dart

@@ -22,9 +22,6 @@ class MeshCoreTcpManager {
   }
 
   Future<void> disconnect() async {
-    if (!_service.isConnected && _service.activeEndpoint == null) {
-      return;
-    }
     _debugLog?.info('TcpManager.disconnect', tag: 'TCP');
     await _service.disconnect();
   }

lib/services/tcp_transport_service_native.dart

@@ -175,6 +175,11 @@ class TcpTransportService {
   }
 
   void _handleSocketDone() {
+    if (_status == TcpTransportStatus.disconnecting ||
+        _status == TcpTransportStatus.disconnected) {
+      return;
+    }
+    _addFrameError(StateError('TCP socket closed by remote endpoint'));
     unawaited(disconnect());
   }