SessionLock: assert valid session id string

The default string representation of a QUuid is enclosed in curly
braces. dBusServiceNameForSession() calls QStringView::mid() assuming
such a default UUID string representation. One of the callers of this
function - SessionLock::handleLockedSession() - does always pass the
result of a QUuid::toString() call. The other two callers of
dBusServiceNameForSession() apparently assume the same session id string
format.

QStringView::mid() will be ported to QStringView::sliced() once KDevelop
depends on Qt 6. The behavior is undefined if the second argument to
sliced() is negative.

Assert the string format assumption in dBusServiceNameForSession() to
better document it and catch a potential bug earlier.