test: 为内存积分服务添加获取余额的模拟方法

添加 get_balance 方法到 _InMemoryPointsService 模拟类中，以便在测试中能够模拟查询用户余额的操作。

danding_bot/plugins/group_horse_racing/test_commands.py

@@ -190,6 +190,10 @@ class _InMemoryPointsService:
         self.calls.append(("refund_bet_points", {"user_id": user_id, "amount": amount, "reason": reason}))
         return True, 0
 
+    async def get_balance(self, user_id: str) -> int:
+        self.calls.append(("get_balance", {"user_id": user_id}))
+        return 0
+
 
 class _NoopMessageService:
     def clear_pending_recalls(self, scope: str):