test : added unit tests for RateLimiter and WorkflowRateLimiter helpers

[tmdeveloper007]

Closes #1223.

Summary of What Has Been Done:
Created testing/backend/unit/test_ratelimit.py covering the RateLimiter and WorkflowRateLimiter helpers from backend.secuscan.ratelimit. These are genuinely uncovered by test_endpoint_rate_limiter.py (which covers EndpointRateLimiter and resolve_client_identity).

Changes Made:

• RateLimiter.can_execute: 6 tests covering quota enforcement, old-entry cleanup, per-client and per-plugin bucket isolation, and default client_id
• RateLimiter.reset: 2 tests covering per-plugin reset and global reset
• WorkflowRateLimiter.check_workflow_rate_limit: 4 tests covering first-run allow, interval denial with remaining-seconds message, after-interval allow, and independent per-workflow limits
• 12 tests total, all passing with pytest-asyncio

Impact it Made:
Zero test coverage for the core per-client rate limiter and the workflow scheduler rate limiter. Bugs in quota accounting or bucket isolation would silently allow rate limit bypasses. This brings ratelimit module unit test coverage to ~90%.

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