Fix CreateMessageRule.Role to be of type Role (#264)

tests/ModelContextProtocol.AspNetCore.Tests/Utils/TestServerTransport.cs

@@ -72,7 +72,7 @@ public class TestServerTransport : ITransport
         await WriteMessageAsync(new JsonRpcResponse
         {
             Id = request.Id,
-            Result = JsonSerializer.SerializeToNode(new CreateMessageResult { Content = new(), Model = "model", Role = "role" }),
+            Result = JsonSerializer.SerializeToNode(new CreateMessageResult { Content = new(), Model = "model", Role = Role.Assistant }),
         }, cancellationToken);
     }