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

tests/ModelContextProtocol.AspNetCore.Tests/HttpServerIntegrationTests.cs

@@ -206,6 +206,8 @@ public abstract class HttpServerIntegrationTests : LoggedTest, IClassFixture<Sse
     [Fact]
     public async Task Sampling_Sse_TestServer()
     {
+        Assert.SkipWhen(GetType() == typeof(StatelessServerIntegrationTests), "Sampling is not supported in stateless mode.");
+
         // arrange
         // Set up the sampling handler
         int samplingHandlerCalls = 0;