Redirect registration flow to login flow

We have a custom registration flow in which we can pre-populate certain registration information based on a passed in URL parameter. In the event a user tries to access this same URL after they’ve already registered, we can detect that and would like to forward them directly to the login page.

My intent was to do this from FormAction.buildPage() but I do not see any way to redirect the user from either FormContext or LoginFormsProvider objects. This seems like a really basic use case. Am I missing something obvious here?

I am looking for this same functionality. I need to redirect using the FormContext.