Issue 21 jpa test coverage

[springframeworkguru]

This pull request finalizes and documents the completion of comprehensive JPA repository test coverage for the openespi-common module, introduces improvements to test infrastructure and guidelines, and makes minor enhancements to the codebase for robustness. The most important changes are grouped below by theme.

JPA Test Coverage Completion & Documentation

• Marked all tasks in the Phase 3 implementation plan, task list, and quality assurance checklist as complete, confirming 100% repository test coverage, advanced testing scenarios, and documentation deliverables. This includes 25+ repository test classes, 300+ test methods, and full integration into CI/CD with coverage reporting. (.junie/prompts/Issue-21-jpa-test-coverage/task.md, [1]; .junie/prompts/Issue-21-jpa-test-coverage/phase3-plan.md, [2]
• Added a requirements draft and updated working prompts to guide the requirements improvement and planning process for JPA test coverage. (.junie/prompts/Issue-21-jpa-test-coverage/requirements-draft.md, [1]; .junie/prompts/Issue-21-jpa-test-coverage/working-prompts.md, [2]

Test Infrastructure & Dependency Enhancements

• Added the DataFaker library as a test dependency in pom.xml for generating realistic test data in repository tests. (openespi-common/pom.xml, openespi-common/pom.xmlR355-R362)
• Updated the Maven Surefire plugin configuration to support Mockito agent and JVM argument for improved test isolation and mocking. (openespi-common/pom.xml, openespi-common/pom.xmlR620-R623)

Codebase Robustness Improvements

• Updated CustomerEntity to initialize statements and phoneNumbers lists to empty ArrayList instances, preventing potential null pointer exceptions during test and production usage. (openespi-common/src/main/java/org/greenbuttonalliance/espi/common/domain/customer/entity/CustomerEntity.java, [1] [2]

Project Guidelines & Best Practices

• Refined .junie/guidelines.md by removing verbose test and build command sections, focusing on high-level testing and build instructions, and adding a best practice for JPA persistence tests (do not set the ID property manually). (.junie/guidelines.md, [1] [2] [3]

These changes collectively ensure robust, maintainable, and well-documented JPA repository test coverage, with supporting infrastructure and guidelines for future development.

Closes #21