test : add unit tests for cacheGet and cacheSet functions

[tmdeveloper007]

Summary of What Needs to be Done:
Add unit tests for the cacheGet and cacheSet functions in src/lib/metrics-cache.ts. Tests must use vitest framework and import actual source code.

Changes that Need to be Made:

• Create test using vitest describe/it/expect
• Import cacheGet and cacheSet from src/lib/metrics-cache.ts
• Test cacheGet returns null when redis unavailable
• Test cacheSet validates TTL values
• Test round-trip when cache available

Impact that it would Provide:

• Ensures cache handles failure gracefully
• Validates TTL input validation

[github-actions]

✅ Closed as completed!

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