Add stateless Streamable HTTP support (#392)

- It allows a single MCP session spanning multiple requests
  to be handled by different servers without sharing state
- It does require the servers share data protection keys, but this
  is standard for ASP.NET Core cookies and antiforgery as well
- It immediately throws for unsupported operations like sampling

src/ModelContextProtocol.AspNetCore/HttpMcpServerBuilderExtensions.cs

@@ -26,6 +26,7 @@ public static class HttpMcpServerBuilderExtensions
         builder.Services.TryAddSingleton<StreamableHttpHandler>();
         builder.Services.TryAddSingleton<SseHandler>();
         builder.Services.AddHostedService<IdleTrackingBackgroundService>();
+        builder.Services.AddDataProtection();
 
         if (configureOptions is not null)
         {