Migrated the test suite from Jest (via tsdx) to Vitest, aligning with React 19 testing best practices and modern tooling.

Changes

packages/formik

• Added Vitest dependencies: vitest, @vitest/ui, @vitest/coverage-v8, jsdom, @testing-library/jest-dom
• Created vitest.config.ts with jsdom environment, globals enabled, and coverage configuration
• Updated test/setupTests.ts to import from vitest and filter React act() warnings
• Replaced Jest APIs throughout test files:
  • jest.fn() → vi.fn()
  • jest.spyOn() → vi.spyOn()
  • jest.Mock → vi.Mock
  • fdescribe → describe
• Updated test scripts: test, test:watch, test:ui, test:coverage
• Removed Jest configuration from package.json

packages/formik-native

• Added Vitest with minimal configuration

Root

• Removed @types/jest dependency
• Updated CONTRIBUTING.md references from Jest to Vitest

Before/After

- "test": "tsdx test --env=jsdom"
+ "test": "vitest run"
+ "test:coverage": "vitest run --coverage"

- import { render } from '@testing-library/react';
+ import { vi } from 'vitest';
+ import { render } from '@testing-library/react';

- const validate = jest.fn();
+ const validate = vi.fn();

Results

• 227 tests passing (226 formik + 1 formik-native)
• 88.36% coverage maintained
• ~4-5s test execution with Turbo caching support

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