refactor

[tomwangsvc]

1. Refactored InvoiceService to improve payment processing logic and clarity.
2. Added robust handling for zero-amount invoices and missing invoice references.
3. Separated logic for invoices with and without previous payments.
4. Implemented clear return messages for all payment scenarios (e.g., fully paid, partially paid, overpayment, no payment needed).
5. Updated payment application to correctly update AmountPaid, calculate TaxAmount, and persist changes.