#249 chore(test): migrate from Enzyme to React Testing Library

Summary

• Replace enzyme and enzyme-adapter-react-16 with @testing-library/react for React 18 compatibility
• Update react-test-renderer to v18
• Add TextEncoder polyfill for Node 18+ jsdom compatibility
• Migrate all existing tests to RTL idioms
• Add additional test coverage (45 tests total, 98%+ coverage)

Changes

Dependencies

• Added: @testing-library/react, @testing-library/jest-dom, @testing-library/user-event
• Removed: enzyme, enzyme-adapter-react-16
• Updated: react, react-dom, react-test-renderer to v18

Test Infrastructure

• New setupTests/rtl.js with TextEncoder polyfill and jest-dom matchers
• Updated jest.config.js to use setupFilesAfterEnv

Test Coverage

• Axis restrictions (x, y, none)
• Min/max constraints
• All callbacks (onResize, onResizeStart, onResizeStop)
• All 8 handle directions
• Event.persist handling
• Component unmounting
• Edge cases (no callbacks, no children)

Test plan

• All 45 tests pass
• Coverage at 98%+
• ESLint passes (1 minor warning)
• Flow has expected test-file ref nullability errors (non-blocking)