feat: ai, slack test 코드 작성

[hyeeuncho]

📝 작업 내용

이번 PR에서 작업한 내용을 간략히 설명해 주세요. (마이크로서비스 명시)

• [ai-service] test 코드 작성
• [slack-service] test 코드 작성

[gemini-code-assist]

Code Review

This pull request introduces comprehensive unit tests for AiMessageService, AiService, SlackMessageService, and SlackService to verify core business logic, status transitions, and role-based access control. The review feedback focuses on improving code maintainability and readability by replacing hardcoded "MASTER" role strings with the UserRole enum, adding the necessary imports, and using static imports for Mockito's ArgumentMatchers instead of fully qualified names.