Add comprehensive unit test suite with Jest for Next.js + Amplify app

[bigweavergammainternal]

This pull request was generated by @kiro-agent 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
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Problem

The amplify-next-template repository has no testing framework or unit tests, leaving the codebase without automated quality assurance. This gap makes it harder to catch regressions, validate behavior of Amplify integrations, and maintain confidence during future development.

Solution

Set up a complete Jest-based testing infrastructure and add comprehensive unit tests covering all key modules in the project.

Testing Framework Setup

• Jest with ts-jest transform for TypeScript/JSX support
• jsdom test environment for React component testing
• @testing-library/react and @testing-library/jest-dom for idiomatic component assertions
• Module name mappers for CSS, static files, and @/ path aliases
• Transform ignore patterns configured to allow @aws-amplify and aws-amplify modules
• jest.setup.ts for global test matchers
• Mocks for CSS (styleMock.ts) and static files (fileMock.ts)

Unit Tests Added

Module	File	Coverage
App Component (app/page.tsx)	__tests__/app/page.test.tsx	Rendering, initial state, observeQuery subscription & state updates, createTodo via prompt, UI interactions
RootLayout (app/layout.tsx)	__tests__/app/layout.test.tsx	Children rendering, Inter font className, lang attribute, metadata export validation
Data Resource (amplify/data/resource.ts)	__tests__/amplify/data/resource.test.ts	Schema definition with Todo model, content field, publicApiKey authorization, defineData config with apiKey auth mode
Auth Resource (amplify/auth/resource.ts)	__tests__/amplify/auth/resource.test.ts	defineAuth invocation, email-based login configuration
Backend Definition (amplify/backend.ts)	__tests__/amplify/backend.test.ts	defineBackend called with correct auth and data resources

Additional Files

• Dockerfile and .dockerignore for containerized test execution
• npm test and npm run test:coverage scripts added to package.json
• Testing devDependencies added: jest, ts-jest, jest-environment-jsdom, @testing-library/react, @testing-library/jest-dom, @types/jest

Notes

• This does not modify any existing application or Amplify backend source code
• This does not include integration tests or end-to-end tests; scope is limited to unit tests
• AWS Amplify modules (generateClient, defineAuth, defineData, defineBackend) are fully mocked to isolate unit test behavior from cloud services
• package-lock.json is not updated in this PR; dependencies should be installed via npm install after merge
• The Dockerfile references base-custom-mcp-image:latest as a base image for containerized test runs