chore: migrate to Vitest v4

[aminevg]

Summary

• Upgrade vitest from v3 (^3.1.4) to v4 (^4.1.0)
• No configuration or test code changes needed — the project was already using the v4-compatible projects field instead of the deprecated workspace option

Migration guide reference

https://vitest.dev/guide/migration.html#vitest-4

Key breaking changes reviewed (none affect this project):

• workspace → projects: already using projects
• Third-argument options syntax removed: not used
• Pool config consolidation (maxThreads/maxForks → maxWorkers): no custom pool config
• Coverage option changes: no coverage config
• Browser provider changes: no browser testing
• Reporter removals (basic, verbose): not used
• vite-node removal: not referenced directly

Test plan

• All 11 test files / 46 tests pass with pnpm test