Stdio SubscribeToResourceAsync Hangs

[MadEste]

Describe the bug
Client.SubscribeToResourceAsync hangs. I am able to successfully subscribe using SendMessageAsync instead.

To Reproduce
Steps to reproduce the behavior:

// Create a client capable of subscribing to resources
bool resourceUpdated = false;
McpClientOptions clientOptions = new()
{
    Capabilities = new()
    {
        NotificationHandlers = new List<KeyValuePair<string, Func<JsonRpcNotification, CancellationToken, ValueTask>>>
        {
            new KeyValuePair<string, Func<JsonRpcNotification, CancellationToken, ValueTask>>(
                NotificationMethods.ResourceUpdatedNotification, (notification, cancellationToken) =>
                {
                    resourceUpdated = true;
                    return new ValueTask(Task.CompletedTask);
                })
        },
    }
};

await LaunchMcpClient(clientOptions);

// Query the servers resources
Verify.IsNotNull(Client?.ServerCapabilities?.Resources);
Verify.IsTrue(Client!.ServerCapabilities!.Resources!.Subscribe == true);

await Client!.SubscribeToResourceAsync("test://text/resource1");

builder.Services.AddMcpServer(options =>
{
    if (options.Capabilities?.Resources is not null)
    {
        options.Capabilities.Resources.Subscribe = true;
        options.Capabilities.Resources.SubscribeToResourcesHandler = (notification, cancellationToken) =>
        {
            if (notification.Params?.Uri is not null)
            {
                subscriptions.Add(notification.Params?.Uri!);
            }

            return new ValueTask<EmptyResult>();
        };

        options.Capabilities.Resources.UnsubscribeFromResourcesHandler = (notification, cancellationToken) =>
        {
            if (notification.Params?.Uri is not null)
            {
                subscriptions.Remove(notification.Params?.Uri!);
            }

            return new ValueTask<EmptyResult>();
        };

    }
    else
    {
        Environment.FailFast("StdioTestServer capabilities were not properly configured.");
    }
})

Additional context
I added subscribe handlers through callback instead of builder extensions because of #656

[hulumane]

@stephentoub We found this issue while testing WMCP