test: add Vitest setup and <hot-header> behaviour tests (Issue #150)

[Raghavcr7]

Summary

• Set up Vitest + jsdom + Testing Library.
• Added tests/setupTests.ts with polyfills/mocks:
  • ElementInternals polyfill
  • wa-* element stubs
  • Mocks for theme injector, Hanko, and icons.
• Added tests/components/header.spec.ts covering:
  • Element registration
  • size="small" class
  • selectTab() logic and guards
  • "tab-change" event detail
  • Login flow (dialog + event).
• All tests pass locally.

Next steps

• Add Playwright visual regression setup (starting with ).

Ref: #150

[spwoodcock]

Thanks for the input!

How much of this is written by you, and how much is AI generated?
(we do have a policy for LLM code at HOT, but can be flexible if its a mix)

Please be honest 🙏

[spwoodcock]

Can you comment for why we need polyfills here? 🙏

[Raghavcr7]

My bad, I had added the polyfills and mocks earlier just to test the logic and make sure the Vitest setup was working properly in jsdom.
I meant to remove them afterward and thought I already had, but looks like I missed that.
They’re no longer needed I just reran everything, and the tests pass cleanly without both. I’ll remove them in the next commit to keep things tidy.

[Raghavcr7]

Also, just to be transparent I wrote around 80% of the code myself.
I used AI mostly to help debug issues, refine TypeScript configs, and test small snippets

[spwoodcock]

Thanks for the heads up! Sounds good =)