Migrate active unit tests from Jest to Vitest

[ciampo]

Part of #80855.

What?

TL;DR — test implementation changes: migrate the active package and integration suites from Jest APIs to explicit Vitest imports, mocks, timers, snapshots, and project-scoped environments. Five Jest tooling suites remain temporarily for the later cutover/cleanup PRs.

Why?

This is the main test-conversion step. It preserves existing intent and CI coverage while making Vitest the runner for Gutenberg-owned application, package, and integration tests.

How?

• Convert Jest globals and helpers to explicit vitest imports with globals disabled.
• Keep browser-like tests on jsdom by default and platform/tooling tests on Node.
• Preserve package aliases, CSS/worker/WASM mocks, setup order, console matchers, fake timers, snapshot serializers, source locations, sharding, date/timezone coverage, and Flakiness.io identity.
• Add exact routed-test type checking and require each owning workspace to declare Vitest.
• Migrate tests added on trunk during the stack rebase, including Editor autosave, Sync, Widget Primitives, and icon-resolution coverage.
• Review snapshot changes individually; no bulk snapshot regeneration is used as evidence.

Testing Instructions

1. Run npm run test:unit:routing.
2. Run npm run test:unit:conventions.
3. Run npm run test:unit -- --runInBand.
4. Run all four test:unit:vitest shards.
5. Run npm run lint:js -- --quiet, npm run lint:pkg-json, npm run lint:lockfile, and npm run lint:tsconfig.

Testing Instructions for Keyboard

Not applicable; this changes tests and infrastructure, not product behavior.

Use of AI Tools

Codex assisted with implementation, conflict reconciliation, and verification. The resulting changes and test output were reviewed by the author.

[github-actions]

Size Change: +12 B (0%)

Total Size: 7.78 MB

📦 View Changed

Filename	Size	Change
build/scripts/compose/index.min.js	11.6 kB	+2 B (+0.02%)
build/scripts/editor/index.min.js	501 kB	+10 B (0%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 58ecf75.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/30582519123
📝 Reported issues:

• [Flaky Test] sort patterns #79882 in /test/e2e/specs/site-editor/patterns.spec.js