test : added unit tests for scorePrMergeRate function

[tmdeveloper007]

Closes #628.

Summary of What Has Been Done:
Added unit tests for scorePrMergeRate function.

Changes Made:

• Created test/repo-health.test.js with tests for scorePrMergeRate function

Impact it Made:

• Improves test coverage for utility functions

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

Two issues with this PR:

1. Missing EOF newline — test file must end with \n

2. Tests don't import actual source code — some tests copy-paste the implementation inside the test file (e.g., #662 redefines getRateLimitRemainingFromResponse locally). Tests must import and test the real function from src/. Reimplementing the function in the test only verifies the copy, not the actual code.

3. No test runner configured — the project has no jest, vitest, or test script in package.json. Please add vitest (or jest) config and a test script so these tests can actually run in CI. Reference: project uses TypeScript so vitest with @vitest/ui is the simplest fit.

Please fix all three and push again.