test: Achieve 95% line coverage with comprehensive unit tests

[devin-ai-integration]

Achieve 95% Line Coverage with Comprehensive Unit Tests

Summary

This PR improves unit test coverage from 2% to 95%+ for the BankApp banking application, following the Java Unit Testing playbook with JUnit 5 and JaCoCo.

Key Changes:

• Migrated existing test from JUnit 4 to JUnit 5
• Added 46 new unit tests across service and controller layers:
  • BankingServiceImplTest (17 tests) - Core business logic for customer/account operations
  • BankingServiceHelperTest (13 tests) - Entity ↔ DTO conversion methods
  • AccountControllerTest (4 tests) - Account & transaction endpoints
  • CustomerControllerTest (5 tests) - Customer CRUD endpoints
• Configured JaCoCo with 95% line coverage and 85% branch coverage enforcement
• Updated pom.xml with JUnit 5, Mockito, AssertJ, and maven-surefire-plugin

Coverage Results:

• Service layer: 100% line coverage
• Controller layer: 100% line coverage
• Helper layer: 100% line coverage
• Overall: 95%+ (model/domain DTOs excluded per playbook guidance)

Review & Testing Checklist for Human

⚠️ Critical - Manual verification required:

• Run the application locally and verify it still starts correctly (mvn spring-boot:run)
• Test key API endpoints via Swagger UI (http://localhost:8989/bank-api/swagger-ui.html):
  • Create a customer (POST /customers/add)
  • Add an account (POST /accounts/add/{customerNumber})
  • Perform a transfer (PUT /accounts/transfer/{customerNumber})
  • Verify the transfer appears in transaction history
• Review JaCoCo exclusions in pom.xml - confirm that excluding model/domain packages is appropriate for this project
• Decide on TODO_COVERAGE.md - This appears to be a planning document that was committed; should it remain in the repo or be removed?
• Run full test suite locally (mvn verify) to confirm all 47 tests pass and coverage gates are met

Notes

• All tests use mocked dependencies (repositories, services) - they verify logic correctness but not actual database interactions
• Integration tests are out of scope for this PR per the playbook's focus on unit testing
• Model/domain classes (DTOs, entities) are excluded from coverage as they contain minimal logic (mostly getters/setters/builders via Lombok)
• No production code was modified - only test infrastructure and new test files added

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Link to Devin run: https://app.devin.ai/sessions/c2674c33b26c4e29bfed0cf18b6c66aa
Requested by: Jaime Mizrachi (@jaime-leo)

[devin-ai-integration]

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