Add comprehensive new test cases across all layers

[devin-ai-integration]

Summary

Adds 11 new test files with ~60 new test cases that significantly expand coverage across all layers of the application. These target gaps identified in the existing test suite including completely untested endpoints, missing edge cases, domain model unit tests, and authorization logic.

New Test Files

Unit Tests (Core/Domain):

• UserTest — User creation, selective field updates, null handling, unique ID generation
• ArticleUpdateTest — Article update logic, slug regeneration, tag deduplication, timestamp behavior
• AuthorizationServiceTest — Article/comment write authorization for authors, comment owners, and unauthorized users
• UtilTest — isEmpty() edge cases (null, empty, whitespace, non-empty)

API Tests (Controller Layer):

• TagsApiTest — GET /tags endpoint (previously had zero API-level tests)
• UsersApiLoginEdgeCaseTest — Login with non-existent email, blank fields, invalid email format; registration with blank/missing fields
• ProfileApiEdgeCaseTest — 404 for non-existent profiles, follow/unfollow non-existent users, 401 without auth
• ArticleApiEdgeCaseTest — Read as authenticated user, 404 on update/delete non-existent articles, 401 without auth, partial update
• CommentsApiEdgeCaseTest — 404 on non-existent article/comment, 401 without auth, article author deleting another user's comment
• ArticleFavoriteApiEdgeCaseTest — 401 without auth, 404 on non-existent articles
• ListArticleApiFilterTest — Filtering by tag/author/favorited, pagination params, combined filters, feed pagination

Review & Testing Checklist for Human

• Verify all 11 new test files compile and pass (./gradlew test)
• Spot-check that test assertions match expected API behavior (especially authorization and 404 edge cases)
• Confirm no existing tests were modified — all changes are purely additive

Notes

• All tests follow the existing patterns (RestAssured + MockMvc for API tests, plain JUnit for unit tests)
• Tests pass locally alongside all existing tests with ./gradlew test

Link to Devin session: https://app.devin.ai/sessions/0f1e4b42fdb74defa8b6b2bb3393595a
Requested by: @scottyandrade99

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[devin-ai-integration]

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