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

まず、現在 ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(jsonrpcResponse) を返しているサーバーパスを追跡し、次に受信した CallToolRequest の処理箇所を特定します。issue に基づいて、SSE へのアップグレード、進捗通知、最終的な CallToolResponse、およびストリームのクローズを定義し、既存の server-response テストがこのシーケンスをカバーしているか確認します。