test : added unit tests for CacheClient helpers

[tmdeveloper007]

Closes #1075.

Summary of What Has Been Done:
Added testing/backend/unit/test_cache.py covering the CacheClient class in backend.secuscan.cache. The class implements an in-memory TTL cache with LRU eviction. 15 tests, all passing.

Changes Made:

• Created testing/backend/unit/test_cache.py
• _sweep_expired: removes expired keys, preserves non-expired keys
• _evict_lru: evicts LRU entries when over capacity, no-op when under capacity
• get_json: returns None for missing/expired keys, updates access order, returns stored value
• set_json: stores value with correct TTL, uses settings.cache_ttl_seconds as default, triggers eviction when at capacity with a new key
• delete_prefix: removes all keys matching a given prefix, handles empty results
• size property: returns entry count
• stats property: reports eviction_count, sweep_count, size, max_entries

Impact it Made:
Zero test coverage for the cache module. CacheClient is used throughout the API for response caching. Untested, TTL bugs could cause stale data to persist indefinitely and LRU eviction bugs could cause unbounded memory growth.

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

[utksh1]

Requesting changes. The tests mutate the global cache settings object directly and do not restore it, which can leak state into later tests. Please use monkeypatch or a local Settings override and keep the cache tests isolated.