fix(app_check,web): fix an error that could occur when refreshing a token (#18135)

Author: Lyokone

packages/firebase_app_check/firebase_app_check_web/lib/firebase_app_check_web.dart

@@ -171,9 +171,18 @@ class FirebaseAppCheckWeb extends FirebaseAppCheckPlatform {
           _delegate!.idTokenChangedController?.close();
         },
       );
-      _delegate!.onTokenChanged(app.name).listen((event) {
-        _tokenChangesListeners[app.name]!.add(event.token.toDart);
-      });
+      _delegate!.onTokenChanged(app.name).listen(
+        (event) {
+          _tokenChangesListeners[app.name]!.add(event.token.toDart);
+        },
+        // Forward JS SDK errors (e.g. network failures during background
+        // token refresh) to the broadcast controller instead of letting them
+        // surface as unhandled zone errors. If nobody is listening on the
+        // broadcast stream the error is silently dropped.
+        onError: (Object error) {
+          _tokenChangesListeners[app.name]?.addError(error);
+        },
+      );
     }
   }