Add JUnit 5 tests for 5 least-covered service/API classes

[devin-ai-integration]

Summary

Adds unit tests for the 5 least-covered classes in the service layer and API exception handling, identified via JaCoCo coverage analysis. All tests are additive — no production code was modified.

Baseline coverage: 33% instruction coverage → 37% after these tests (110 tests total, up from 68).

New Test Class	Target Class	Prior Coverage	Pattern
ArticleCommandServiceTest (7 tests)	ArticleCommandService	6.7%	Mockito @Mock/@InjectMocks
CommentQueryServiceCursorTest (9 tests)	CommentQueryService	51.2%	Mockito, focuses on findByArticleIdWithCursor
CustomizeExceptionHandlerTest (7 tests)	CustomizeExceptionHandler	63.4%	Direct instantiation + mocked exceptions
UserServiceTest (8 tests)	UserService	71.4%	Mockito with mocked PasswordEncoder
CursorPagerTest (11 tests)	CursorPager	73.3%	Plain unit tests, no mocks

Each test class covers happy path, error cases, and edge cases (e.g., null user, empty lists, nested constraint violation paths).

Review & Testing Checklist for Human

• ArticleCommandServiceTest.should_create_article_with_null_tag_list asserts NullPointerException — this documents current behavior where null tag list causes an NPE in Article constructor. Verify this is acceptable rather than masking a bug that should be fixed with graceful handling.
• UserServiceTest mocks bypass Spring's @Validated — the @UpdateUserConstraint validator logic (UpdateUserValidator) is never exercised by these tests. The tests verify User.update() delegation but not duplicate-email/username validation. This is by design for unit tests but worth noting.
• CustomizeExceptionHandlerTest does NOT cover handleMethodArgumentNotValid (the @Override method). The handler has 4 exception-handling methods but only 3 are tested. Consider whether this gap matters.
• Run ./gradlew test jacocoTestReport locally and verify all 110 tests pass. The build will report a JaCoCo verification failure (37% < 80% threshold) which is expected — overall coverage is still dragged down by the untested GraphQL layer.

Notes

• The @SuppressWarnings("unchecked") usage in CustomizeExceptionHandlerTest is necessary due to Mockito's generic type limitations when mocking ConstraintViolation<?> and ConstraintDescriptor<?>. The doReturn().when() pattern is used instead of when().thenReturn() to work around wildcard capture issues.
• Overall project coverage remains at 37% (below the 80% JaCoCo threshold) primarily because the GraphQL datafetcher/mutation layer (~12 classes) has near-zero coverage and was out of scope for this PR.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/2bbc0116e91f42988ece07e935d863af

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[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.