test : added unit tests for cache module-level functions

[tmdeveloper007]

Closes #1261.

Summary of What Has Been Done:
Added a new test file testing/backend/unit/test_cache_module.py covering the four module-level functions in cache.py that were not tested in the existing test_cache_helpers.py (which focuses on CacheClient).

Changes Made:

• New file: testing/backend/unit/test_cache_module.py
• TestInitCache: 3 test cases — returns CacheClient, sets global, replaces existing global
• TestGetCache: 2 test cases — returns initialised cache, raises RuntimeError when not initialised
• TestInvalidateViewCache: 2 test cases — no-op when cache is None, calls delete_prefix for each expected prefix
• TestInvalidatePluginCaches: 2 test cases — no-op when cache is None, calls delete_prefix for summary: and plugins: prefixes
• ASCII only, no unicode, trailing newline at end of file

Impact it Made:

• These module-level functions are called by route handlers after every write (findings, reports, plugins). Incorrect prefixes cause stale cache to persist and corrupt API responses.
• No existing tests covered these functions.

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