Streamable Stateless server should support upgrade to SSE response

Expected Behavior

A server that is fully stateless and does not support long-lived connections can still take advantage of streaming in this design.
For example, to issue progress notifications during a tool call:
When the incoming POST request is a CallToolRequest, server indicates the response will be SSE
Server starts executing the tool
Server sends any number of ProgressNotifications over SSE while the tool is executing
When the tool execution completes, the server sends a CallToolResponse over SSE
Server closes the SSE stream

https://github.com/modelcontextprotocol/modelcontextprotocol/pull/206

Current Behavior

The current implementation can only return json

return ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(jsonrpcResponse);

Context

首先跟踪当前返回 ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(jsonrpcResponse) 的服务器路径，然后定位对传入 CallToolRequest 的处理。根据 issue 定义 SSE 升级、进度通知、最终的 CallToolResponse 以及流关闭，并检查现有的 server-response 测试是否覆盖这一序列。