fix(gotrue): prevent double completion of refresh token completer

Check if the completer is already completed before calling completeError
in dispose() to avoid "Bad state: Future already completed" errors during
concurrent dispose operations.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: grdsdev

packages/gotrue/lib/src/gotrue_client.dart

@@ -1351,7 +1351,10 @@ class GoTrueClient {
     _onAuthStateChangeControllerSync.close();
     _broadcastChannel?.close();
     _broadcastChannelSubscription?.cancel();
-    _refreshTokenCompleter?.completeError(AuthException('Disposed'));
+    final completer = _refreshTokenCompleter;
+    if (completer != null && !completer.isCompleted) {
+      completer.completeError(AuthException('Disposed'));
+    }
     _autoRefreshTicker?.cancel();
   }