test : added unit tests for rate limiter classes

[tmdeveloper007]

Closes #1076.

Summary of What Has Been Done:
Added testing/backend/unit/test_ratelimit.py covering RateLimiter and ConcurrentTaskLimiter in backend.secuscan.ratelimit. 11 tests, all passing.

Changes Made:

• Created testing/backend/unit/test_ratelimit.py
• RateLimiter: allows within quota, blocks when exhausted, tracks per-client-per-plugin independently, uses global as default client_id, reset(plugin_id) clears only that plugin, reset(None) clears all buckets
• ConcurrentTaskLimiter: acquire succeeds under limit, fails at limit, release frees a slot, release of unknown task is a no-op, get_available_slots returns correct count

Impact it Made:
Zero test coverage for the rate limiting module. These classes protect the backend from runaway task spawning and enforce per-client rate limits. Untested, a bug in the sliding window or concurrent slot logic could silently allow DoS or resource exhaustion.

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.