Flow ExecutionContext with JsonRpcMessage (#616)

The primary goal of this change is to support IHttpContextAccessor in tool calls when the Streamable HTTP is in its default non-Stateless mode.

src/ModelContextProtocol.Core/Server/StreamableHttpPostTransport.cs

@@ -91,6 +91,11 @@ internal sealed class StreamableHttpPostTransport(StreamableHttpServerTransport
 
         message.RelatedTransport = this;
 
+        if (parentTransport.FlowExecutionContextFromRequests)
+        {
+            message.ExecutionContext = ExecutionContext.Capture();
+        }
+
         await parentTransport.MessageWriter.WriteAsync(message, cancellationToken).ConfigureAwait(false);
     }
 }