Payment method to use for following payments during and post reactivation.
If unset, the subscription's existing default_payment_method is preserved.
Behaviour by collection_method:
- CHARGE_AUTOMATICALLY: a fresh invoice is finalized and immediately auto-charged using this PM
(or the existing default if unset). Reactivation requires a non-empty PM.
- REQUEST_PAYMENT: a fresh invoice is finalized but NOT auto-charged; the customer pays it
out-of-band. The PM is optional here and stored on the subscription for future flips/use.