fix(be): mock redis and object mapper for controller tests

[beekeeper24]

• mock Redis 커넥션/템플릿을 주입해서 JwtAuthenticationFilter 테스트 시 NPE 방지 (실제 Redis 연결이 없어서 터지던 문제 해결)
• 테스트 전용 ObjectMapper에 JavaTimeModule + KotlinModule 등록해 ZonedDateTime 직렬화 오류 해결

[Copilot]

Pull Request Overview

Fixes NullPointerException in controller tests by mocking Redis dependencies and resolves ZonedDateTime serialization errors with proper ObjectMapper configuration.

• Adds comprehensive mocking for Redis connection and template to prevent NPE during JWT filter tests
• Configures test ObjectMapper with JavaTimeModule and KotlinModule for proper ZonedDateTime serialization
• Changes entity ID fields from val to var to support test data manipulation

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
TestConfig.kt	Adds Redis mocks and ObjectMapper configuration for tests
ChatRoom.kt	Changes ID field from immutable to mutable for test flexibility
ChatMessage.kt	Changes ID field from immutable to mutable for test flexibility

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[YangHJ2415]

수고하셨습니다!