Cache authorization policy lookups for IMcpServerPrimitives

The authorization filters added by #733 looks up policies for IMcpServerPrimitives every request. This used to also be done by the authorization middleware, but .NET 7 introduced IAuthorizationPolicyProvider.AllowsCachingPolicies which makes it possible to cache at least some authorization policy. We could add an Items dictionary to IMcpServerPrimitives to track the cached policy similar to the Items dictionary #733 adds to the RequestContext<TParams>, or we could use a ConditionalWeakTable<IMcpServerPrimitive, AuthorizationPolicy?>.

Inizia dai filtri di autorizzazione aggiunti da #733 ed esamina IMcpServerPrimitives, RequestContext e IAuthorizationPolicyProvider.AllowsCachingPolicies. Confronta gli approcci proposti del dizionario Items e di ConditionalWeakTable, quindi verifica che i criteri di autorizzazione idonei vengano riutilizzati tra le richieste senza modificare il comportamento dell’autorizzazione.