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

Comienza trazando la ruta del servidor que actualmente devuelve ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(jsonrpcResponse), y luego localiza el manejo de una CallToolRequest entrante. Define, a partir del issue, la actualización a SSE, las notificaciones de progreso, la CallToolResponse final y el cierre del flujo, y comprueba si las pruebas existentes de respuestas del servidor cubren esta secuencia.