test: cover leaderboard cache edge cases (#1278)

[mittalsonal]

Summary

• add exact-boundary expiry coverage for �pruneExpiredLeaderboardCache�
• cover null-entry, far-future expiry, and primitive-payload preservation cases
• keep the change scoped to �test/leaderboard-cache.test.mjs� only

Testing

• �node --test test/leaderboard-cache.test.mjs�
• �git diff --check�

Closes #1278

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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

[mittalsonal]

Maintainer label request: if this looks good, please add the GSSoC labels including gssoc:approved and the appropriate difficulty/type labels so it counts correctly on the platform. Thanks!

[mittalsonal]

Verification note:

• node --test test/leaderboard-cache.test.mjs
• git diff --check

I kept this PR intentionally test-only and limited to leaderboard cache edge-case coverage so it stays easy to review and merge.