Add stateless doc, prefer stateless mode and disable legacy SSE by default (#1468)

samples/AspNetCoreMcpPerSessionTools/Program.cs

@@ -13,6 +13,10 @@ PopulateToolDictionary(toolDictionary);
 builder.Services.AddMcpServer()
     .WithHttpTransport(options =>
     {
+        // This sample demonstrates per-session tool filtering, which requires stateful mode.
+        // Set Stateless = false explicitly for forward compatibility in case the default changes.
+        options.Stateless = false;
+
         // Configure per-session options to filter tools based on route category
         options.ConfigureSessionOptions = async (httpContext, mcpOptions, cancellationToken) =>
         {