QQmlTypeLoader::createNetworkAccessManager() creates a new QNAM on every request by default

The default implementation of QQmlTypeLoader::createNetworkAccessManager returns a  new QNetworkAccessManager on every request, if the application developer does not set the factory as described in the documentation (https://doc.qt.io/qt-6/qqmlnetworkaccessmanagerfactory.html):

QNetworkAccessManager *QQmlTypeLoader::createNetworkAccessManager(QObject *parent) const
{
    // Can be called from both threads, or even from a WorkerScript

    // TODO: Calling the user's create() method under the lock is quite rude.
    //       However, we've been doing so for a long time and stopping the
    //       practice would expose thread safety issues in user code.
    // ### Qt7: Maybe change the factory interface to provide a different method
    //          that can be called without the lock.
    if (const auto factory = QQmlNetworkAccessManagerFactoryPtrConst(&m_data))
        return factory->create(parent);

    return new QNetworkAccessManager(parent);
}

As vminenko correctly found on QTBUG-136048, this is not in line with what we suggest in the documentation of QNetworkAccessManager, where we write:

"One QNetworkAccessManager instance should be enough for the whole Qt application. Since QNetworkAccessManager is based on QObject, it can only be used from the thread it belongs to." https://doc.qt.io/qt-6/qnetworkaccessmanager.html

It seems reasonable to have a default implementation that follows our own documentation and reduces the numbers of QNetworkAccessManagers in apps where application developers do not explicitly take care of it.

Having a single QNetworkAccessManager instance would further simplify resetting it, if that is required e.g. through changing network connections.