DollarPay compatibility update - 2026-10-13

- Optional return_url is omitted if it exceeds DollarPay's 128-byte limit.
  Signed callbacks and status checks remain the payment authority.
- Documented creation/query responses may omit transaction_id. Checkout
  accepts a valid signed response and HTTPS checkout URL. Settlement still
  checks the merchant order reference, merchant ID and exact amount.
  Conflicting transaction IDs, when supplied, are rejected.
- Successful DollarPay responses must pass signature verification.
- DollarPay callback acknowledgement is exactly SUCCESS. XXPayUS and
  PixPayment continue to receive lowercase success.
- Rejections include a safe status code and configuration hint in order
  diagnostics. Raw response messages, buyer data and credentials are not logged.
- Existing payment attempts are never automatically retried or cleared.

Validated locally with dummy notebook orders and simulated signed responses.
Live merchant credentials, channel activation and provider permission for
fees/splits must be tested independently. Existing amount rules are unchanged.
