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/Protocol/Transport/StreamableHttpClientSessionTransport.cs

@@ -57,7 +57,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         cancellationToken = sendCts.Token;
 
 #if NET
-        using var content = JsonContent.Create(message, McpJsonUtilities.DefaultOptions.GetTypeInfo<JsonRpcMessage>());
+        using var content = JsonContent.Create(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
 #else
         using var content = new StringContent(
             JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage),