When a service worker sends an HTTP request or when accessing navigator.userAgent the default Qt user agent is used instead of the one set by the user with QWebEngineProfile::setHttpUserAgent()

Client hints seems to also behave the same, always using default values instead of the one set by the user.

To reproduce:

1. Take simple browser demo
2. Add m_profile->setHttpUserAgent("foobar"); to Browser::createHiddenWindow(bool offTheRecord)
3. Run simplebrowser
4. Go to youtube.com
5. Open dev tools
6. Open the console and type navigator.userAgent, check that the value is "foobar"
7. Change the context of the console to "sw.js" using the combobox in the toolbar of the console
8. Type navigator.userAgent, the value is Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) QtWebEngine/6.9.2 Chrome/130.0.0.0 Safari/537.36 instead of foobar