fix(contexts): detect the browser's implicit context and create targets in it without id - #627

…ts in it without id

Connecting to a browser that came up with its own startup window, raised `Failed to find browser context with id` on the first `create_page`. That window lives in the browser's implicit context, which Chrome below 145 refuses to address by id.

We now detect the implicit context up front as the context holding targets that `Target.getBrowserContexts` doesn't report, since that reports only contexts created through `Target.createBrowserContext`, and create targets in it the way Chrome expects, by omitting `browserContextId`. `Target.getBrowserContexts` covers every client driving the browser, so a context another Ferrum created in it is never mistaken for the implicit one.

`Ferrum::Context#implicit?` tells the two apart. The implicit context is never disposed, so `#reset` leaves such a
browser's startup window and cookies alone; launch the remote browser with --no-startup-window to get a context of
our own that `#reset` can clean up.

Closes #578