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/IdleTrackingBackgroundService.cs

@@ -12,7 +12,7 @@ internal sealed partial class IdleTrackingBackgroundService(
     ILogger<IdleTrackingBackgroundService> logger) : BackgroundService
 {
     // The compiler will complain about the parameter being unused otherwise despite the source generator.
-    private ILogger _logger = logger;
+    private readonly ILogger _logger = logger;
 
     protected override async Task ExecuteAsync(CancellationToken stoppingToken)
     {