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/MapMcpStreamableHttpTests.cs

@@ -6,6 +6,7 @@ namespace ModelContextProtocol.AspNetCore.Tests;
 public class MapMcpStreamableHttpTests(ITestOutputHelper outputHelper) : MapMcpTests(outputHelper)
 {
     protected override bool UseStreamableHttp => true;
+    protected override bool Stateless => false;
 
     [Theory]
     [InlineData("/a", "/a")]
@@ -22,7 +23,7 @@ public class MapMcpStreamableHttpTests(ITestOutputHelper outputHelper) : MapMcpT
                 Name = "TestCustomRouteServer",
                 Version = "1.0.0",
             };
-        }).WithHttpTransport();
+        }).WithHttpTransport(ConfigureStateless);
         await using var app = Builder.Build();
 
         app.MapMcp(routePattern);