[Sample Request] Payment workflow with idempotency, state machine, and compensation flow

Hi, I'd like to contribute a new sample demonstrating a production-grade payment workflow pattern.

What it would cover:

• 7-state payment state machine (INITIATED → VALIDATING → PROCESSING → COMPLETED / FAILED / CANCELLED / COMPENSATING)
• Idempotency key deduplication before any state mutation
• Exponential backoff with jitter on retries
• Compensation flow as a child workflow on failure

Why it's useful:
The existing HelloSaga and MoneyTransfer samples demonstrate SAGA and basic compensation, but none show the full combination of idempotency + state machine transitions + compensation in a single cohesive example. This pattern is extremely common in fintech/payment backends.

Happy to submit a PR if this is welcome. Let me know if there's a preferred package structure or naming convention I should follow.

Bắt đầu bằng cách đọc các sample HelloSaga và MoneyTransfer hiện có để hiểu cách SAGA và compensation được minh họa. Xác định sample xoay quanh bảy trạng thái được yêu cầu, xử lý idempotency, retry backoff và compensation của child-workflow; công việc hoàn tất khi một payment workflow thống nhất thể hiện cả bốn hành vi.