为什么server转发消息给目的client的时候要走netty而不是直接http调用呢

RT，以clientA发消息给clientB为例，我目前梳理流程是这样的：
clientA ->(http invoke) -> router ->(http invoke) -> server，
然后server转发消息到clientB的时候是查找SessionSocketHolder，找到对应的NioSocketChannel，然后把消息write进去。
所以我的疑问是，为什么客户端发给服务端的请求直接走http，由SpringBoot的Controller受理，而server转发的请求要走netty呢？

Start by tracing the SpringBoot Controller request path and the server-side SessionSocketHolder lookup described in the issue, including where the NioSocketChannel is written. Compare those two paths and document why each transport is used, with a clear explanation of the clientA-to-clientB flow as the definition of done.

Written by the indexing model from the issue text.