Add test coverage for ScanRateLimiter._make_key

[tmdeveloper007]

Problem

ScanRateLimiter._make_key in backend/secuscan/rate_limiter.py builds Redis key strings for per-IP rate-limit buckets. No direct unit tests exist for it.

Fix

1. Extract RateLimiterKeyBuilder.make_key into backend/secuscan/ratelimit_helpers.py.
2. Write 7 unit tests covering IP variants, window types, edge cases, and key format.
3. Update rate_limiter.py to re-export from the new module.

Acceptance Criteria

• New ratelimit_helpers.py created with RateLimiterKeyBuilder.make_key.
• 7 tests pass locally.