FIX: extract reset_last_seen_cache! in user_spec (PR #9808)

Helper method to avoid code duplication


Minor but I prefer keeping this in the model proper, that way you get to reused the key calculation code that the model uses.

I know it can sometimes feel a bit weird to have stuff like this in the model, so this is a bit of a bike shed. But overall this reduces code duplication which is a good thing.