Fix worker targets counted as browser windows

[sean0407]

Summary

window_handles
- before: return every tracked CDP target
+ after: return only IDs for `page` targets

Fixes #326. Ferrum 0.18 tracks page, iframe, worker, shared-worker, and service-worker targets; only page targets are browser windows.

Evidence

• Before: The new worker-window regression example failed on the original implementation: expected 1 handle, got 2.
• After: The regression example and existing window-listing example passed (2 examples, 0 failures). The full spec/features/driver_spec.rb passed (156 examples, 0 failures, 6 pending).
• bundle exec rubocop lib/capybara/cuprite/browser.rb spec/features/driver_spec.rb: 2 files inspected, no offenses.

Merge Danger

Door: Two-way

Blast Radius: Low

Only window-handle enumeration changes; non-page CDP targets are excluded while page targets and existing popup behavior remain covered by specs.

[route]

Thanks, I just prepared mine yesterday #333