Assistant (Qt5): Bug in helpviewer.cpp, tries to display qthelp://-pdf files as text files, QDesktopServices::openUrl mechanism is skipped

With assistant 5.15.3 in Ubuntu22 we cannot open PDF-files from a `qthelp://`-URL. They are either displayed as binary text within the assistant or we get prompted a "policy changed" error (WebKit 102). The root cause is that the QDesktopServices::openUrl mechanism which should handle local URLs like `qthelp://` is never called due to a bug in the code.

The bug is still present in the HEAD version of `helpviewer.cpp`
https://github.com/qt/qttools/blob/dev/src/assistant/assistant/helpviewer.cpp

bool HelpViewer::launchWithExternalApp(const QUrl &url)
{
    TRACE_OBJ
    if (isLocalUrl(url)) {
        const HelpEngineWrapper &helpEngine = HelpEngineWrapper::instance();
        const QUrl &resolvedUrl = helpEngine.findFile(url);
        if (!resolvedUrl.isValid())
            return false;

        const QString& path = resolvedUrl.toLocalFile();
        if (!canOpenPage(path)) {

´helpEngine.findFile` typically does not alter or solve the URL (to a file path) but returns it unaltered. This can also be seen in the docstring of `helpEngine.findFile`:

/*!
    Returns an invalid URL if the file \a url cannot be found.
    If the file exists, either the same url is returned or a
    different url if the file is located in a different namespace
    which is merged via a common virtual folder.
*/

Therefore `resolvedUrl` is a `qthelp://...`-URL and calling `toLocalFile();` on it simply returns an empty string. The empty string will always return `true` for `canOpenPage`. Thus the if-block below is never entered and assistant never calls `QDesktopServices::openUrl` to open the PDF file.

The solution is to fix the code as following:

const QString& path = resolvedUrl.path();