Subscriptions: when re-subscribing with email-unverified account, prompts for code

When trying to re-subscribe via a blog post set to be accessed by "subscribers only", a popup appears to ask for an auth code:

Steps:

- Create an account and don't verify its email https://wordpress.com/start/user-social
- While logged in at WP.com, subscribe at https://cicerosletters.wordpress.com/ (until [fixed](https://github.com/Automattic/jetpack/issues/39434))
- Clear your cookies/login status, and open "subscribers only" post and try re-subscribing from https://cicerosletters.wordpress.com/2023/12/08/free-subscribers-only/
- You'll see auth code flow

Since they're a subscriber already, they should probably go via login flow if their email hasn't been confirmed yet. If their subscription status would be pending, they would need to go "confirm your subscription" email flow.

Login flow also confirms their email. We can either redirect them directly onto the login page but that might be a bit jarring if they forgot they are a subscriber. We could also inform them and offer a link to the login page.

Maybe like so? @crisbusquets

Reproduce the flow using the WordPress.com account-creation, subscription, and subscribers-only URLs in the issue, including the auth-code prompt after clearing cookies. Trace the authentication and subscription paths they trigger; done means an existing subscriber with an unverified email is sent through login, while a pending subscription uses the confirmation-email flow.

Written by the indexing model from the issue text.