I have a Durable Azure Function I just moved over to .NET 8 isolated worker model, from .NET 6 in-proc.

I am struggling with unit testing/mocking return data of type EntityMetadata for DurableEntityClient.GetEntityAsync<T>().

Given the following code i want to test/mock, which is used in my Durable Function:

async Task<MyState?> GetEntityValueAsync(DurableTaskClient client, EntityInstanceId entityId)
{
    // this is the new way to get entity state in .NET 8 isolated 
    var entity = await client.Entities.GetEntityAsync<MyType>(entityId);

    return await entity.State.Get();
}

I would expect to be able to write a unit test like this, mocking the services and setting up the expected return data:

[Theory]
[AutoMockData]
public async Task MyTest(
    [Frozen] DurableTaskClient taskClient,
    MyFunction function,
    MyMessage message,
    MyState state)
{
    // arrange
    var entityId = new EntityInstanceId(nameof(MyType), message.ContextId.ToString());
    var entityState = new EntityMetadata<MyType>(entityId);

    var durableClientMock = new Mock<DurableTaskClient>("test");
    var entityClient = new Mock<DurableEntityClient>("test");

    durableClientMock
        .Setup(d => d.Entities).Returns(entityClient.Object);

    entityClient
        .Setup(d => d.GetEntityAsync<MyType>(It.IsAny<EntityInstanceId>(), It.IsAny<CancellationToken>()))
        .ReturnsAsync(entityState);

    // Act
    // GetEntityValueAsync is called in .DoTheThing() 
    await function.DoTheThing(JsonConvert.SerializeObject(message), taskClient);

    // Assert
    // stuff should work.
}

However, this does not work. When the test runs, and it tries to call:

var entity = await client.Entities.GetEntityAsync<MyType>(entityId);

I get the error:

System.ArgumentException : Can not instantiate proxy of class: Microsoft.DurableTask.Client.Entities.EntityMetadata`1.

Could not find a parameterless constructor. (Parameter 'constructorArguments')

How can I fix this? How should I properly be mocking EntityMetadata responses? It is a sealed class so I cannot extend from it and make my own test implementation.