Refund models — request, response, and OrderResponse integration

[rammrain]

Summary

Define refund DTOs and integrate them into OrderResponse.

Context

Issue #14 implemented OrderResponse but deferred refund-related fields. The Montonio API returns refunds, availableForRefund, and (already included) isRefundableType on the order response. The TypeScript client defines:

interface RefundResponse {
    uuid: string;
    amount: number;
    status: string;
    currency: Currency;
    createdAt: string;
    type: string;
}

Requirements

• Create RefundResponse response DTO in an appropriate package (e.g., sdk.order.response or a new sdk.refund package)
• Add refunds: List<RefundResponse> and availableForRefund: BigDecimal fields to OrderResponse
• Unit tests for serialization/deserialization round-trips

Refs

• OrderResponse in montonio-client: refunds: RefundResponse[], availableForRefund: number
• Issue Payment order models — requests, responses, and enums #14 design doc: docs/plans/2026-04-10-payment-order-models-design.md