Cancel a phone-number purchase or an active rental

Cancels a Mobilerun Phone. The outcome depends on the number's current state:

• If the number is still awaiting payment and no payment for it is currently being processed, the checkout is closed immediately and the number is retired.
• If the number is on the standard paid plan and already paid and in service, cancellation is scheduled for the end of the current billing period rather than taking effect immediately. The number stays usable through the period already paid for, with no partial refund. Calling this again while a cancellation is already scheduled is a no-op that returns the same result. The response's state reflects this as cancel_scheduled with cancelAtPeriodEnd: true; currentPeriodEnd is populated once billing confirms the cancellation.

Any other state (already refunding, a permanent billing failure, a payment currently being processed, an included-plan number, or a non-hosted/BYO number) returns 409 not_cancellable. Returns 404 if the number doesn't exist or isn't owned by the caller.