… to Vitest

• Migrate test file from packages/core/lib/v3/tests/ to packages/core/tests/
• Replace @playwright/test imports with vitest (describe, expect, it)
• Update test.describe() -> describe() and test() -> it()
• Adjust import path for SupportedUnderstudyAction enum

why

what changed

test plan

---

Summary by cubic

Migrated the Zod enum compatibility tests from Playwright to Vitest and moved them to packages/core/tests. This consolidates on Vitest and keeps Zod v3/v4 enum compatibility coverage for SupportedUnderstudyAction.

• Refactors
  • Replaced @playwright/test with vitest (describe, expect, it).
  • Updated test.describe/test() to describe/it.
  • Adjusted SupportedUnderstudyAction import to ../lib/v3/types/private/handlers.