test: add vitest tests for loyalty-rewards-engine.js addEarnedPoints

[tmdeveloper007]

📄 Description

Extended tests/unit/loyalty-rewards-engine.test.js with new describe block for addEarnedPoints covering all tier multipliers (Bronze 1x, Silver 1.25x, Gold 1.5x, Platinum 2x), negative input, NaN guard, and history accumulation.

🔗 Related Issues

Closes #7529

🧩 Type of Change

• Bug fix
• New feature
• Documentation update
• Style / UI improvement
• Refactor
• Test
• Chore / dependency update

✅ Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Note: This task is being handled by tmdeveloper007 as part of GSSOC (GirlScript Summer of Code) — please assign this PR to the tmdeveloper007 account when picking it up.