Add comprehensive testing infrastructure with ~99% coverage

[devin-ai-integration]

Summary

Adds a full testing stack to the ecommerce sample app (which previously had zero test coverage) and achieves 99.41% statement, 97.5% branch, 98.36% function, and 99.33% line coverage.

Testing Infrastructure

• Jest with jsdom environment for unit/integration tests
• React Testing Library (@testing-library/react@12 for React 17 compatibility)
• Playwright for E2E tests
• Mock setup: CSS modules via identity-obj-proxy, image imports via fileMock, Next.js mocks (router, link, head)
• Coverage thresholds: 90% for branches, functions, lines, statements

Test Scripts Added

• npm test — run all unit/integration tests
• npm run test:coverage — run tests with coverage report
• npm run test:e2e — run Playwright E2E tests
• npm run test:all — run both unit and E2E tests

Tests Written (153 tests across 16 suites)

• Components (7 suites): Cart, Footer, FooterBanner, HeroBanner, Layout, Navbar, Product — rendering, interactions, props, conditional rendering, checkout flow
• Pages (3 suites): Home (getServerSideProps), Success (cart reset, confetti), Product Detail (getStaticPaths/getStaticProps, quantity controls, add to cart, buy now)
• API Routes (2 suites): hello (status/response), stripe (session creation, image URL transformation, price conversion, error handling, method validation)
• Context (1 suite): StateContext — add/remove/toggle cart items, quantity controls, boundary conditions
• Lib (3 suites): Sanity client initialization + urlFor, Stripe singleton loader, confetti utility timing
• E2E (4 specs): Homepage, product browsing, cart interactions, success page

Coverage Report

File               | % Stmts | % Branch | % Funcs | % Lines
All files          |   99.41 |     97.5 |   98.36 |   99.33
 components        |     100 |      100 |     100 |     100
 context           |     100 |    91.66 |     100 |     100
 lib               |     100 |      100 |     100 |     100
 pages             |     100 |      100 |     100 |     100
 pages/api         |     100 |      100 |     100 |     100
 pages/product     |      96 |      100 |   88.88 |   95.83

Review & Testing Checklist for Human

• Run npm run test:coverage locally and verify coverage output matches reported numbers
• Review mock implementations in __mocks__/ to ensure they accurately represent the modules they replace (next/router, next/link, next/head)
• Run npm run test:e2e with the dev server to verify E2E tests pass against a real Sanity backend (requires valid .env with Sanity credentials)

Notes

• Used @testing-library/react@12 for React 17 compatibility (project uses React 17.0.2)
• --legacy-peer-deps was needed during install due to peer dependency constraints
• E2E tests require a running dev server with valid Sanity API credentials to pass
• Lint passes with zero errors (existing warnings in source code are pre-existing)
• components/index.js and pages/_app.js are excluded from coverage as they are framework re-exports

Link to Devin session: https://app.devin.ai/sessions/d6e354b4057e4d47ba1194093f08c9e6
Requested by: @davidbean-hash

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