test : added unit tests for repo-health scoring functions

[tmdeveloper007]

Refs #1281.

Summary of What Has Been Done:
Added unit tests for individual scoring functions in src/lib/repo-health.ts: scoreCommitFrequency, scorePrMergeRate, scoreOpenIssuesCount, and scoreDaysSinceLastCommit. Also exported these functions to enable direct testing.

Changes Made:

• Enhanced test/repo-health-scoring.test.ts with 4 new describe blocks covering all scoring functions
• Exported scoring functions in src/lib/repo-health.ts for testability
• Added boundary and edge case tests for each function

Impact it Made:
Improved test coverage for the repository health scoring algorithm. All 30 tests pass.

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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