chore(test): centralize mock definitions

This commit refactors test utilities by moving `MockWebSocketSink` from `web_socket_mocks.dart` to the centralized `mocks.dart` file.

This change avoids duplicating mock definitions and improves the organization of test helper code. The `whenListenWebSocket` function has been updated to use the centrally defined mocks.

Author: xsahil03x

packages/stream_feeds/test/test_utils/mocks.dart

@@ -6,6 +6,8 @@ import 'package:web_socket_channel/web_socket_channel.dart';
 
 class MockDefaultApi extends Mock implements api.DefaultApi {}
 
+class MockWebSocketSink extends Mock implements WebSocketSink {}
+
 class MockWebSocketChannel extends Mock implements WebSocketChannel {}
 
 api.UserToken generateTestUserToken(String userId) {

packages/stream_feeds/test/test_utils/web_socket_mocks.dart

@@ -3,9 +3,8 @@ import 'dart:convert';
 
 import 'package:mocktail/mocktail.dart';
 import 'package:stream_feeds/src/ws/events/events.dart';
-import 'package:web_socket_channel/web_socket_channel.dart';
 
-class _MockWebSocketSink extends Mock implements WebSocketSink {}
+import 'mocks.dart';
 
 /// Sets up WebSocket channel mocks for testing.
 ///
@@ -20,10 +19,10 @@ class _MockWebSocketSink extends Mock implements WebSocketSink {}
 /// whenListenWebSocket(webSocketChannel, wsStreamController);
 /// ```
 void whenListenWebSocket(
-  WebSocketChannel webSocketChannel,
+  MockWebSocketChannel webSocketChannel,
   StreamController<Object> wsStreamController,
 ) {
-  final webSocketSink = _MockWebSocketSink();
+  final webSocketSink = MockWebSocketSink();
 
   when(
     webSocketSink.close,