Back the BrowserKit CookieJar with the browser context

[kbond]

getCookieJar() returns a copy of the browser's cookies that CookieJarSync refills at three points: construction, after a navigation, and after setCookie(). Between them it is stale, so a cookie set by javascript or by a fetch response is invisible until the next navigation. Writes are worse, since nothing propagates the other way: set() and expire() change the copy while the browser keeps sending exactly what it sent before.

This replaces the syncing with a BrowserKit jar backed by the context, in the spirit of Symfony\Component\Panther\Cookie\CookieJar. Both clients hand it to parent::__construct(), so getCookieJar() is live, and CookieJarSync is gone.

One BC break, noted in the changelog: cookies in a CookieJar passed to PlaywrightClient were overwritten by the context's own and never sent, so pre-seeding it did nothing. They are now written into the context.