feat(token_storage): add clearCache method for testing and ensure fresh state in tests

lib/core/token_storage.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 abstract class TokenStorage {
@@ -8,6 +9,13 @@ abstract class TokenStorage {
     return _cachedPrefs!;
   }
 
+  /// Clears the cached SharedPreferences instance.
+  /// This is primarily used for testing to ensure fresh state between tests.
+  @visibleForTesting
+  static void clearCache() {
+    _cachedPrefs = null;
+  }
+
   static Future<bool> haveTokenInStorage() async {
     try {
       final sharedPreferences = await _getPrefs();