Make EverythingServer capable of running either stdio or http

From this comment on #709 by @halter73:

It might be interesting to demonstrate how you can share code for an HTTP and stdio everything server. We could create EverythingServer.Http, EverythingSerever.Stdio and EverythingServer.Core projects. And the Core project could have an AddEverythingMcpHandlers(this IMcpServerBuilder builder, ConcurrentDictionary<string, ConcurrentDictionary<string, byte>> subscriptions) method. It might even make sense to add something like public class SubscriptionCollection : ConcurrentDictionary<string, ConcurrentDictionary<string, byte>> to the core project.

The only thing that couldn't be shared is Program.cs. The Stdio project would use hosted services to manage subscriptions, and the Http project would do the RunSessionHandler logic.

Empieza revisando el proyecto EverythingServer existente y su Program.cs; después, lee la discusión enlazada de #709. Compara el flujo de suscripción del servicio alojado stdio con el flujo de HTTP RunSessionHandler. Se considera terminado cuando la lógica compartida del handler y de la suscripción esté separada en un proyecto core, mientras ambos transportes conservan su respectivo código de startup.