Refund Payments

Processes payment refunds by calling payment providers and creating refund transactions.

Supports two refund methods:

• Automatic refunds: Calls the payment provider's API to process the refund.
• External refunds: Marks payments as refunded without calling providers (for manual provider-side refunds).

Set paymentRefunds.externalRefund = true for external refunds when you've manually processed the refund and only need to update the order's transaction records.

The method also handles restocking inventory and sending customer notifications when specified in sideEffects.