Add comprehensive unit tests for the rate limiter implementation:
- TestRateLimiterBasic: Verify basic token bucket behavior
- TestRateLimiterNoMatchingRules: Requests pass when no rules match
- TestRateLimiterLRUEviction: Cache respects max size limit
- TestRateLimiterPathMatching: Different paths have independent limits
- TestRateLimiterMultipleRulesAllMustPass: Strictest matching rule wins
- TestRateLimiterPerKeyIsolation: Different users have separate quotas
- TestExtractRateLimitKeyIP/IPv6: IP-based key extraction
- TestRateLimitConfigRate/EffectiveBurst/Matches: Config calculations
- TestSendRateLimitResponseHTTP/GRPC: Response format verification
- TestRateLimiterTokenRefill: Token bucket refills over time
