`core`: Feign Implementation Details exposed to clients

The use of the Feign library/framework is an implementation detail and shouldn't concern the consumers of the library.

However, every API call throws runtime exceptions all of which extend Feign's. That they are Runtime does abstract away many concerns. However, the APIs should throw declared exceptions (Runtime or not) so that consumers can properly handle error states, without relying on inspecting the underlying error details.

首先检查核心 API 入口点，以及其基于 Feign 的调用当前公开的运行时异常。定义公共异常边界，并确定调用方应如何处理已声明的错误状态；当 API 消费者不再需要依赖 Feign 异常类型时，即视为完成。

由索引模型根据 Issue 内容生成。