test: migrate PWA browser-scoped handler test to mocks

[lisa0314]

Refs #2639.

Summary

• Migrate the existing PWA browser-scoped handler test to createHandlerMocks() and createMockPuppeteerPage(). It previously launched Chrome even though all four PWA operations were already stubbed.
• Assert that no selected page is requested, each PWA operation receives the exact arguments, and the handlers produce the expected response lines and page-list refresh calls.
• Keep all four real-browser PWA integration tests unchanged, including installation/state/uninstallation, standalone launch, explicit launch URL, and selection recovery after uninstall.

No production code or shared mock helpers are changed.

Testing

• Build and npm run check-format: passed; git diff --check: passed.
• PWA test file: 5/5 passed on both this change and clean main 8939965, using Chrome 152.0.7977.83 on macOS with Node 24.18.0.
• With PUPPETEER_EXECUTABLE_PATH=/nonexistent/pwa-unit-tests/chrome, the migrated test passes; the original test on the same main fails because it tries to launch Chrome.
• Mutation checks: adding a selected-page lookup or passing an incorrect manifest ID makes the migrated test fail; both mutations were reverted and the test passes again.
• Full npm run test on both this change and a clean same-main worktree: 1056 passed, 4 failed, 3 skipped each. Both fail the previous-navigation network snapshot and oversized screenshot tests. Both also have extension-related failures, but the failing extension lifecycle/content-script case differs. A focused rerun of all three extension cases implicated across the two runs passes 3/3 on both worktrees. These are not green full-suite runs.

Rebase verification (2026-09-29)

• Rebasing onto main 47c5b37 preserves a single test-only commit in tests/tools/pwa.test.ts.
• PUPPETEER_EXECUTABLE_PATH=/Applications/Google Chrome.app/Contents/MacOS/Google Chrome npm run test tests/tools/pwa.test.ts: 6/6 passed; npm run check-format and git diff --check: passed.
• The full-suite results above are from earlier bases; the full suite was not rerun on 47c5b37.