Migrate testing infrastructure from Jest (CRA) to Vitest

[devin-ai-integration]

Summary

Migrates the testing infrastructure from Jest (via Create React App) to Vitest (Session C of 4-session parallel migration).

Changes:

• Removed old testing libraries from dependencies: @testing-library/react (^12), @testing-library/jest-dom (^5), @testing-library/user-event (^13)
• Installed new testing dependencies:
  • vitest (^2), jsdom, @vitejs/plugin-react (^4)
  • @testing-library/react (^16), @testing-library/jest-dom (^6), @testing-library/user-event (^14)
  • @testing-library/dom (peer dependency of @testing-library/react v16)
• Created vitest.config.js with:
  • Path aliases matching CRA's jsconfig-style imports
  • jsdom test environment and global test APIs
  • esbuild configured to handle JSX in .js files (CRA convention)
  • react-dom/client alias to a compatibility shim for React 17
• Added src/test/react-dom-client-shim.js — bridges @testing-library/react v16's dependency on react-dom/client (React 18 API) to work with React 17's ReactDOM.render. Can be removed once React is upgraded to v18+.
• Updated src/setupTests.js — removed CRA boilerplate comments
• Updated package.json test script: "test": "vitest"
• Fixed src/App.test.jsx — replaced broken "learn react" test with a "renders without crashing" test wrapped in AlertProvider context

Note on version compatibility

@testing-library/react v16 requires React 18+, but this project currently uses React 17. A react-dom/client shim is provided to bridge this gap. Once React is upgraded to v18+ (potentially via another parallel migration session), the shim file and the react-dom/client alias in vitest.config.js can be removed.

Review & Testing Checklist for Human

• Run npx vitest run to verify the test passes
• Verify that the react-dom/client shim adequately bridges React 17 ↔ @testing-library/react v16
• Check that path aliases in vitest.config.js match the project's import conventions
• After merging other parallel migration PRs, verify no conflicts in package.json

Notes

• Only test-related files were modified (App.test.jsx, setupTests.js, vitest.config.js, package.json). No component source files or index.js were touched.
• Used Vitest v2 (with Vite 5) instead of Vitest v4 (Vite 8) because Vite 8's oxc parser does not support JSX in .js files, which is a CRA convention used throughout this codebase.
• --legacy-peer-deps was required during install due to the React 17 / @testing-library/react v16 peer dependency mismatch.

Link to Devin session: https://app.devin.ai/sessions/a59ac023896c4ff68a1e13d94f593e3e
Requested by: @shahmir-masood

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