Create vendor refund payment with allocations

Create a vendor refund payment with embedded allocations in a single atomic operation.

This endpoint follows the same pattern as invoice payments, where allocations are embedded in the payment creation request rather than being separate API calls.

Key Features:

• Atomic operation - all or nothing
• Comprehensive validation
• Support for multiple allocation types
• Idempotency support via Idempotency-Key header

Allocation Types:

• bill: Apply refund to a specific bill
• payment: Refund a specific bill payment
• credit: Create a credit balance for future use

Idempotency: Include an Idempotency-Key header to ensure the same request isn’t processed twice. If the same key is used again, the original response will be returned.

Example Request:

{
  "vendor_id": "uuid",
  "total_amount_cents": 100000,
  "payment_method": "bank_transfer",
  "refund_date": "2025-01-15",
  "allocations": [
    {
      "allocation_type": "bill",
      "bill_id": "uuid",
      "amount_cents": 60000,
      "description": "Partial refund for Bill #123"
    },
    {
      "allocation_type": "credit",
      "amount_cents": 40000,
      "description": "Credit for future purchases"
    }
  ]
}