Fix session isolation for remote browser WebSocket connections

[wooly]

When using browser_ws_endpoint to connect to a remote Chrome instance, pages are created via browser.newPage() in the default browser context. Cookies (and other browsing state) set during one request persist in the default context and leak into subsequent requests from other users.

This PR updates the behaviour to use browser.createBrowserContext() instead. This behaves like an incognito session — state is fully isolated and automatically cleared when context.close() is called after the request completes.

Added a test that:

1. Launches a persistent Chrome instance (simulating a remotely running Chrome) and retrieves its WS endpoint
2. Makes a first request that sets a cookie via evaluateOnNewDocument
3. Makes a second request with no cookies and asserts the cookie from the first request is not visible

Without the fix, the second request sees session-secret=abc123 (bleed). With the fix it sees no cookies.

[abrom]

A few Rubocop lints that needed addressing, but otherwise looks great. Nice one @wooly 👍

[abrom]

Released in v1.2.10