test(utils): Enhance unit test coverage for getViewerLocalHour with invalid date strings

[Rakshak05]

Description

Fixes #8633

This PR adds comprehensive unit test coverage for the getViewerLocalHour utility function in utils/dateHelpers.ts.

Changes Included:

• Added unit tests for:
  • Valid ISO strings with various timezone offsets (+02:00, -05:00, Z).
  • Partial ISO date strings (2024-03-10, 2024-03).
  • Empty strings ('').
  • Malformed date strings with invalid characters (invalid-date, 2024-03-XX, hello world).
  • Incomplete / out-of-range ISO strings (2024-03-10T, 2024-13-99T25:99:00).
  • Invalid ancient dates before year 0 (-9999999-01-01).
  • Out-of-bounds future dates (999999999-01-01).
  • Safe handling of non-string / missing values (null, undefined).
• Added type guard validation in getViewerLocalHour (typeof isoDate !== 'string') for added runtime safety.
• Ensured deterministic test execution across timezones in CI using bounded range assertions (0 to 23).

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Bug fix, refactoring, docs)

Visual Preview

N/A (Unit test coverage expansion)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
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[retenta-bot]

This PR adds significant unit test coverage for the getViewerLocalHour function, addressing various invalid date scenarios. This aligns with previous decisions to enhance testing around timezone normalization and calendar boundary handling, ensuring our date calculations remain robust across different contexts. Great work on this! Looking forward to your next contributions.

[github-actions]

📦 Next.js Bundle Size Report (Gzipped Sizes)

✨ No significant bundle size changes detected.

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	5004.34 KB	5004.32 KB	+0.02 KB (+0.00%)
Total CSS	354.80 KB	354.80 KB	0 B

[Aamod007]

Solid set of edge case tests! The extra type guard on \isoDate\ is a great small addition for runtime safety. Approved!