test: add unit tests for achievement-estimators helper functions

[yachikadev]

Closes #2794

Summary

Added unit tests for the pure helper functions calculateNextTier and calculatePercentage in src/lib/achievement-estimators.ts.

Changes

• Created test/achievement-estimators.test.ts with full coverage for both functions

Test Cases Covered

calculateNextTier

• Returns first tier when current is below all tiers
• Returns next tier when current is below a tier
• Returns next tier when current equals a tier
• Returns null when current exceeds all tiers (maxed out)
• Returns null for empty tiers array
• Returns first tier when current is negative

calculatePercentage

• Returns 100 when nextTier is null (maxed out)
• Returns 100 when current exceeds nextTier
• Returns floor of ratio when current is below nextTier
• Returns 100 when current equals nextTier
• Handles 0 current gracefully

Notes

Added // @vitest-environment node to the test file since these are pure functions with no DOM dependency, which also fixes a jsdom/undici incompatibility with Node.js v24.

[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

[yachikadev]

The Playwright smoke test failure is not related to this PR.
It is caused by a pnpm-lock.yaml lockfile mismatch with the current overrides config in package.json — a pre-existing repo issue. All checks relevant to this PR pass successfully.