🧪 Add unit tests for formatSecondsToDuration

[ManupaKDU]

This PR adds comprehensive unit tests for formatSecondsToDuration in dateUtil.ts.

It covers:

• Handling of NaN and negative durations.
• Handling of 0 seconds.
• Formatting of simple minutes and seconds cases.
• Formatting of more complex durations involving hours, minutes, and seconds.

The tests ensure code behavior is properly validated in case of future refactoring.

---

PR created automatically by Jules for task 6378999889320003937 started by @manupawickramasinghe

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[github-actions]

PR template validation notice

The PR description could not be validated against the pull request template.

Items to review:

• Missing section: ## Proposed change
• Missing section: ## Type of change
• Missing section: ## AI disclosure
• Missing section: ## Checklist
• The Proposed change section is empty. Please describe what this PR does.

Please consider updating your PR description to include all required sections from the template.