test : added edge case tests for leaderboard-cache pruneExpiredLeaderboardCache

[tmdeveloper007]

Refs #1278.

Summary of What Has Been Done:
Added edge case unit tests for the pruneExpiredLeaderboardCache function in src/lib/leaderboard-cache.ts, covering boundary timestamps, null handling, and type safety.

Changes Made:

• Created new test file: test/leaderboard-cache.test.ts (replacing .mjs version)
• Added tests for boundary timestamp expiry (exactly at timestamp)
• Added tests for entry expiring 1ms before current timestamp
• Added tests for null entry handling
• Added tests for far future expiry times
• Added tests for different generic type parameters

Impact it Made:
Improved test coverage for cache expiration logic. All 8 tests pass.

[vercel]

@TESTPERSONAL is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[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