(feat) O3-5175: Add service-layer validation for Bill and BillLineItem entities

[UjjawalPrabhat]

Description

This PR implements service-layer validation for the Bill and BillLineItem entities in the OpenMRS Billing Module, addressing O3-5175 and bringing the module into compliance with OpenMRS architectural standards.

Problem Statement

The billing module currently lacks proper service-layer validation:

• BillServiceImpl.validate() and BillLineItemServiceImpl.validate() are empty methods
• Validation only enforced at database level via Hibernate constraints
• Users receive cryptic database errors instead of business-friendly messages
• Validation happens too late in the request lifecycle (database layer only)

Solution

Implemented comprehensive service-layer validation following OpenMRS standards:

BillLineItemValidator (5 validation rules):

• At least one of item OR billableService must be set
• Quantity must be positive (> 0)
• Price must not be negative (>= 0)
• Payment status must be valid (PENDING or PAID)
• Field lengths comply with database constraints

BillValidator (6 validation rules):

• Bill must have at least one line item
• All line items must pass BillLineItemValidator
• Patient is required
• Cashier is required
• CashPoint is required
• Valid status and payment coverage for PAID bills

Testing

• 18 comprehensive test cases covering all validation rules
• 90%+ code coverage
• All edge cases covered (null values, boundary conditions, invalid enums)
• All tests passing

Files Changed

New (4):

• BillLineItemValidator.java
• BillValidator.java
• BillLineItemValidatorTest.java
• BillValidatorTest.java

Modified (3):

• BillServiceImpl.java
• BillLineItemServiceImpl.java
• messages_en.properties

Compliance

• Uses Spring Validator interface (OpenMRS standard)
• Handler annotation for auto-registration
• ValidateUtil.validate() invocation pattern
• Follows UserValidator reference pattern
• Backward compatible - no API changes
• All existing tests pass

Related issue: O3-5175
https://openmrs.atlassian.net/browse/O3-5175