using Xunit;
using NSubstitute;
using FluentAssertions;

// ...

    [Fact] 
    public async Task Call_activity()
    {
        YourAcvitity activity = new(/*dependencies*/);
        // since WorkflowActivityContext only has an internal constructor
        // we need to use reflection to instantiate it
        TaskActivityContext inner = Substitute.For<TaskActivityContext>();
        var context = typeof(WorkflowActivityContext)
            .GetConstructor(
                BindingFlags.NonPublic | BindingFlags.Instance,
                null,
                [typeof(TaskActivityContext)],
                null
            )
            .Invoke([inner]) as WorkflowActivityContext;

        var result = await activity
            .RunAsync(
                context,
                new TInput() // whatever your activity uses
            );

        result.Should().NotBeNull();
    }