[Fix] McpRateLimiter의 windows 맵이 TTL 없이 무한정 커지는 문제 수정

[kangcheolung]

Summary

• McpRateLimiter의 windows 맵을 ConcurrentHashMap → Caffeine 캐시(expireAfterAccess 2분)로 교체
• 한 번이라도 호출된 사용자·도구 조합이 서버 재시작 전까지 메모리에 영구히 쌓이던 문제 해소
• checkLimit()의 rate limit 로직(고정 윈도우 판단/리셋/증가) 자체는 변경 없음
• #117에 알려진 한계로 남아있던 이슈를 해소 (신규 발견 아님)

closes #292

Test plan

• checkLimit_evictsEntry_afterTtlExpires 신규 단위 테스트 추가
• 기존 McpRateLimiter 테스트 6개(동시성/레이스 포함) 회귀 확인
• ./backend/gradlew -p backend test 전체 통과

문서

• docs/design/kangcheolung-#292-mcp-rate-limiter-ttl.md

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📥 Commits

Reviewing files that changed from the base of the PR and between b2487fc and 8843754.

📒 Files selected for processing (4)

• backend/build.gradle
• backend/src/main/java/com/opensource/docgrid/domain/mcp/security/McpRateLimiter.java
• backend/src/test/java/com/opensource/docgrid/domain/mcp/security/McpRateLimiterTest.java
• docs/design/kangcheolung-#292-mcp-rate-limiter-ttl.md

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