test : added unit tests for consistency-score calculation functions

[tmdeveloper007]

Closes #2696.

Summary of What Has Been Done: Added unit tests for the pure calculation functions exported from src/lib/consistency-score.ts. Exported six previously-private helper functions to enable isolated unit testing.

Changes Made:

• Exported scoreToGrade, getImprovementTip, computeLongestGap, hasActivityInLastNDays, computeWeeklyConsistency, and computeMonthlyTrend from src/lib/consistency-score.ts
• Created test/consistency-score.test.ts with 24 test cases covering grade boundaries, tip thresholds, gap calculation, activity window detection, weekly consistency scoring, monthly trend computation, full score calculation, and recent-activity flag

Impact it Made: Consistency scoring drives user-visible grades and improvement tips. Test coverage here prevents regressions in scoring logic that would affect developer gamification and streak tracking.

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — 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 things before this can merge:

1. Hard conflict with test : added unit tests for consistency-score.ts isRecentlyActiveFromScore #2762 (already merged): both PRs create test/consistency-score.test.ts as a new file. Please rebase onto main and drop the isRecentlyActiveFromScore describe block entirely — test : added unit tests for consistency-score.ts isRecentlyActiveFromScore #2762 already covers it with stronger assertions (exact formula boundary conditions vs your typeof-boolean check).

2. Keep the rest: the five new describe blocks (scoreToGrade, getImprovementTip, computeLongestGap, hasActivityInLastNDays, computeWeeklyConsistency/Monthly, calculateConsistencyScore) are genuinely new coverage and worth merging. The six export additions to the source file are also fine.

Rebase + remove the duplicate block and this is good to go.