Add unit tests for Payment, ScheduledPayment, and Billing services

[AdithaBuwaneka]

• Implement comprehensive tests for PaymentRepository including save, find, update, and delete operations.
• Create tests for ScheduledPaymentRepository to validate scheduled payment functionalities.
• Develop unit tests for BillingServiceImpl covering invoice creation, payment processing, and scheduled payment management.
• Ensure proper exception handling and authorization checks in service methods.
• Validate payment and scheduled payment retrieval based on various criteria such as customer ID and status.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch Payment-service-mvn-test

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