Summary

• keep uncaptured expect.extend() from installing custom matchers that shadow built-in matcher names on the base expect
• preserve captured extended expect and mergeExpects() behavior for built-in-name overrides
• add regressions for captured, uncaptured, and merged built-in matcher override paths

Fixes #41365.

Tests

• npm run build
• npm run ttest -- tests/playwright-test/expect.spec.ts --grep "(expect.extend should not override builtin matchers|expect.extend should fall back|expect.extend should be immutable|mergeExpects with builtin matcher overrides|should support mergeExpects$)"
• npm run ttest -- tests/playwright-test/expect.spec.ts
• npm run tsc -- --pretty false
• npx eslint packages/playwright/src/matchers/expect.ts tests/playwright-test/expect.spec.ts
• npm run lint-tests