fix: 查询实例列表时支持按 clusters 参数过滤

[lly835]

修复问题

修复 #210：subscribe 以及 selectOneHealthyInstance 传递 clusters 参数后未生效的问题。

原因分析

Service::get_instance_list 方法接收了 cluster_names 参数（变量名为 _cluster_names，带下划线前缀），但方法体内直接调用 get_all_instances 返回所有实例，完全忽略了 clusters 过滤条件。

修复方案

当 cluster_names 非空时，在遍历实例时增加 cluster_names.contains(&x.cluster_name) 过滤条件，只返回属于指定 cluster 的实例；当 cluster_names 为空时保持原有行为，返回所有实例。

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