fix: guard completer against race condition in web connect

[jin123456bat]

Problem

When a WebSocket opens successfully but later encounters an error (e.g., network drops), both onOpen and onError fire on the same connect() call. The second handler attempts to complete an already-completed Completer, causing a StateError: Bad state: Future already completed.

This is commonly seen in Flutter web apps using web_socket_client via pusher_client_socket, where the WebSocket connection is used for real-time communication.

Root Cause

The connect() function listens to both onOpen.first and onError.first with unawaited futures. On the web, a WebSocket can fire both events sequentially (open then error on disconnect), and the second event will crash because the Completer was already resolved by the first.

Fix

Add if (!completer.isCompleted) guards to both onOpen and onError handlers so that whichever event fires second is silently dropped instead of throwing StateError.

Before

socket.onOpen.first.then((_) {
  completer.complete(socket);
});

socket.onError.first.then((event) {
  completer.completeError(error);
});

After

socket.onOpen.first.then((_) {
  if (!completer.isCompleted) completer.complete(socket);
});

socket.onError.first.then((event) {
  if (!completer.isCompleted) {
    completer.completeError(error);
  }
});

Impact

• Eliminates StateError: Bad state: Future already completed noise in production error tracking
• No behavioral change: the first event still wins, the second is silently dropped
• Backward compatible: no API changes