test: add tests for rate_limiter.check_scan_rate_limit (closes #2446)

[tmdeveloper007]

Summary

Adds unit tests for backend.secuscan.rate_limiter.check_scan_rate_limit,a FastAPI dependency that enforces per-tenant scan-rate limits.

Changes

• New test file: testing/backend/unit/test_rate_limiter_check_scan_rate_limit.py
• 4 async tests covering: call through when limiter is set,
  HTTPException propagation, no-op when limiter is None,
  no-op when app.state is absent

Testing

• All 4 tests pass with python -m pytest testing/backend/unit/test_rate_limiter_check_scan_rate_limit.py --noconftest -v

[utksh1]

The code change is reviewed and the backend/lint checks are green. Merge remains blocked by the required frontend audit, which reports the pre-existing high-severity undici vulnerability; I am not bypassing that security gate.