fix(browser): list only page targets as window handles

[route]

Fixes #326

Ferrum 0.18 tracks worker, shared_worker and service_worker targets in the same map as pages, so window_handles reported a page's web workers as windows and window_opened_by raised opened 2 windows instead of 1.

• window_handles returns only page targets
• switch_to_window and close_window raise Ferrum::NoSuchPageError for non-page targets instead of opening a page session on a worker
• regression spec with a fixture that spawns a dedicated worker on load and opens a target=_blank tab; it fails on main with expected: 1, got: 2