Make Preview override work on node add page

Don't have a solution yet, but I think I understand why this is behaving differently from the edit form. If I monitor the tempstore (the key_value_expire table) as I preview from node edit, each preview action overwrites the previous entry for that UUID. If I do the same from the node add form, each preview action gets keyed under a new UUID.

What isn't clear to me is if this is being caused by a flaw in our logic, or is a side effect of the entity not really existing until the entity is initially created.

I don't think it's a flaw. The node hasn't been created yet so it hasn't been persisted with a UUID. I could route around for this but I suspect that there is logic that checks for the existence of a UUID and if one isn't found it generates a new one.

Maybe we could check the tempstore for the last inserted item

I was able to fix this by making the ajax callback a little more dynamic.

Looks good. Didn't manually test but good progress.