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

Inizia tracciando il percorso del server che attualmente restituisce ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(jsonrpcResponse), quindi individua la gestione di una CallToolRequest in ingresso. Definisci, sulla base dell’issue, l’upgrade SSE, le notifiche di avanzamento, la CallToolResponse finale e la chiusura dello stream, quindi verifica se i test esistenti delle risposte del server coprono questa sequenza.