Fix cascading message SagaId propagation with Separated mode

[scrodde]

When a saga handler returns a cascading message, the Sagaid wasn't being set on the MessageContext, so outbound messages had no SagaId attached. This caused IndeterminateSagaStateIdException when the cascaded message tried to correlate back to the saga.

Workaround 1
Explicitly set the Saga identity on the cascaded message by any of the supported ways documented here https://wolverinefx.net/guide/durability/sagas.html#saga-message-identity

Workaround 2
Publish directly using IMessageContext.PublishAsync and using DeliveryOptions { SagaId }

public async Task Handle(ThingUpdated updated, IMessageContext context)
{
    Updates++;
    Debug.WriteLine(context.Envelope.Destination);
    await context.PublishAsync(new AnotherThingHappened(), new()
    {
        SagaId = Id.ToString()
    });
}

Solution

The SagaId extracted from the incoming message wasn't being set on MessageContext._sagaId, so when cascading messages were enqueued the outbound envelope had no SagaId.

Changes:

1. Added SetSagaId(object sagaId) method to MessageContext to allow generated code to set the saga ID for correlation
2. Created SetSagaIdFrame - generates context.SetSagaId(sagaId) for existing saga handler paths where sagaId variable is available
3. Created SetSagaIdFromSagaFrame - generates context.SetSagaId(saga.Id) for Start method paths where only the saga object is available