Migrate jest tets to vitest

[katinthehatsite]

Related issues

Fixes STU-1207

Proposed Changes

This PR migrates from jest to vitest.

Testing Instructions

• Confirm all the tests are passing in the CI
• Review vitest config and setup files and feel free to add suggestions
• You can also run the tests locally to confirm everything is passing correctly

Note: I initially wanted to split the setup files for node and dom but after doing that, I noticed that they had quite a bit in common for now so decided to leave as is. We can also do the split if we think it is necessary.

In some cases, it was tricky to mock things so the mocks might be longer, especially since vitest is quite strict with enforcing types.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[fredrikekelund]

I took a quick look at Buildkite to compare unit test times between this PR and #2465.

Looks like migrating to Vitest basically cut them in half 🤩

	macOS	Windows
Vitest	2min21s	5min51s
Jest	4min30s	8min56s

[wpmobilebot]

📊 Performance Test Results

Comparing 6e64da8 vs trunk

site-editor

Metric	trunk	6e64da8	Diff	Change
load	2894.00 ms	2907.00 ms	+13.00 ms	🔴 0.4%

site-startup

Metric	trunk	6e64da8	Diff	Change
siteCreation	7068.00 ms	7107.00 ms	+39.00 ms	🔴 0.6%
siteStartup	3925.00 ms	3921.00 ms	-4.00 ms	🟢 -0.1%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change

[bcotrim]

Amazing work! @katinthehatsite
All 1,138 tests pass without any errors.

A couple of notes:

1. Some logs from Studio CLI are polluting the test outputs. We could consider mocking ora globally in vitest.setup.ts or adding it to the alias config.

2. There are still some references to "Jest" in some files. For good measure we should probably clean these up:
   • CLAUDE.md:47
   • docs/ai-instructions.md
   • packages/eslint-plugin-studio/ - still uses Jest (could be a follow-up)

[katinthehatsite]

Thanks for taking time to do the review @bcotrim ! All great suggestions, I will take a look at them today 👀

[katinthehatsite]

@bcotrim I made some further changes based on your suggestions, let me know what you think.

[katinthehatsite]

I forgot to mention:

packages/eslint-plugin-studio/ - still uses Jest (could be a follow-up)

Let's do this as a follow-up because there are lots of changes in this PR already 😓 I will open an issue for this

[katinthehatsite]

Created a follow up issue https://linear.app/a8c/issue/STU-1268/migrate-packageseslint-plugin-studio-to-vitest

[bcotrim]

Thanks for addressing the changes and creating the follow-up issue @katinthehatsite!
Sorry for being picky, but there are still some warning logs during test execution. Could we address those before merging?

[katinthehatsite]

Thanks for addressing the changes and creating the follow-up issue @katinthehatsite!
Sorry for being picky, but there are still some warning logs during test execution. Could we address those before merging?

No worries at all, looking 👀

[katinthehatsite]

I made some further updates, let me know what you think!