- Add credit_management_tests.rs with tests for SubscriberAccount, UsageEvent, UsageType, AccountStatus, and integer arithmetic operations
- Add error_tests.rs with tests for ChargingError Display implementation, cloning, and log_error function
- Add rating_billing_tests.rs with tests for ChargingRule variants, as_str conversion, billing period parsing, timestamp filtering, and cost calculations
- Add validation_tests.rs with tests for validate_bytes, validate
