Improve article validation and test coverage

[leesunghyun]

Summary

• Add validation UX to article detail editing (required title/body, inline validation text, disabled save when invalid)
• Add/expand UI tests for back-without-save and validation behavior
• Add reducer unit test and logic guard for invalid article selection IDs
• Keep documentation/checklist state aligned with the actual demo replacement status

Changes

1) Article detail UX (validation)

• app/src/main/java/com/example/viewmodelmigration/ui/ArticleDetailScreen.kt
  • Added required-field checks for title/body
  • Added per-field error state and supporting validation text
  • Added global validation hint text
  • Disabled save button until both fields are valid

2) Reducer safety and tests

• app/src/main/java/com/example/viewmodelmigration/core/ArticleListReducer.kt
  • SelectArticle now validates requested article id exists before setting selection
• app/src/test/java/com/example/viewmodelmigration/core/ArticleListReducerTest.kt
  • Added selectArticle_invalidId_clearsSelectionForInvalidStateSafety

3) Compose UI smoke coverage

• app/src/androidTest/java/com/example/viewmodelmigration/ArticleUiSmokeTest.kt
  • Added saveButton_isDisabledAndShowsValidation_whenRequiredFieldsAreEmpty
  • Added backWithoutSave_keepsOriginalArticleInList

4) Docs / release alignment

• README.md: removed demo placeholder note and updated v0.2 checklist
• docs/release-notes-v0.2.0-oss-alpha.1.md: updated demo-planning wording
• docs/roadmap-v0.2.0.md: marked demo replacement as complete
• docs/images/demo.gif: replaced placeholder with real emulator demo recording

Verification

• Scope reviewed and tests added in source and UI test suites
• ./gradlew test (via GitHub Actions: Android CI)
• ./gradlew assembleDebug (via GitHub Actions: Android CI)
• Emulator validation for smoke paths (manual)

[leesunghyun]

Remaining follow-up: manual smoke validation is still marked unchecked in PR verification. I recommend running a quick 10~20 second emulator smoke pass (edit/save/back cases) and then checking this item manually.

[leesunghyun]

실기기(R5CN30C32XB, SM_G9860)에서 수동 스모크 시나리오 검증 완료했습니다.

검증 절차:

1. 앱 설치 후 첫 번째 항목 편집 -> Title에 SMOKECHECK 추가 -> Save 탭
2. 목록으로 복귀 후 첫 번째 항목 타이틀/바디 반영 확인
3. 두 번째 항목 편집 -> Body에 NOSAVECHK 추가 -> Back 탭
4. 목록에서 두 번째 항목이 변경되지 않은 상태 유지 확인

결과:

• Save 반영: → (텍스트 반영됨)
• Back without save: 항목은 바디 변경 없음(변경 미반영)

PR 체크리스트 [ ] 항목은 실기기 기준으로 완료 처리 가능합니다.

[leesunghyun]

실기기 검증 결과 보완 공지:

• 저장 케이스: 첫 번째 항목 제목이 "Legacy ViewModel"에서 "Legacy ViewModel SMOKECHECK"로 변경되어 목록 반영됨.
• 미저장 뒤 뒤로 가기: 두 번째 항목 "StateFlow Migration"은 편집 중 입력값 NOSAVECHK를 추가했지만 목록으로 복귀 시 바디/제목 변경 없음.
• 따라서 PR 본문의 남은 체크 항목(Emulator validation for smoke paths)은 실기기 기준 완료 처리해도 됩니다.