fix: OAuth 응답에 isRegistered 추가

[lsy1307]

관련 이슈

작업 내용

• 로그인 응답과 회원가입 준비 응답에 isRegistered를 포함합니다.
• 변경된 응답 계약에 맞게 OAuth 서비스 테스트를 변경했습니다.

특이 사항

리뷰 요구사항 (선택)

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✨ Finishing Touches

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