Cancellation must be VERIFIED-IDEMPOTENT: adapters treat an already-terminal
subscription as success (re-fetching on a cancel rejection where needed), so
a replayed cancel can never fail an adoption flow that depends on stopping
PSP-side billing exactly once.
Cancellation must be VERIFIED-IDEMPOTENT: adapters treat an already-terminal subscription as success (re-fetching on a cancel rejection where needed), so a replayed cancel can never fail an adoption flow that depends on stopping PSP-side billing exactly once.