[UWP] `GenerateElementProjection()` performs risky cast in default case

[paulcam206]

AdaptiveCards/source/uwp/AdaptiveCardsObjectModel/lib/ObjectModelUtil.cpp

Lines 313 to 317 in 61677f2

	case CardElementType::Unknown:
	default:
	return winrt::make<winrt::AdaptiveCards::ObjectModel::Uwp::implementation::AdaptiveUnsupportedElement>(
	std::AdaptivePointerCast<AdaptiveCards::UnknownElement>(baseElement));
	}

In the above, if we encounter an element that isn't covered in a case, but is a known element (shouldn't happen, but see #6393), we can end up incorrectly casting a known element to AdaptiveCards::UnknownElement, leading to undefined behavior later on. We should probably split the default case out on its own and throw an exception (and do a quick examination to see if this pattern is repeated elsewhere).