FIX: ensures generic onebox has width/height for thumbnail (#23040)

Prior to this fix we would output an image with no width/height which would then bypass a large part of `CookedProcessorMixin` and have no aspect ratio. As a result, an image with no size would cause layout shift.

It also removes a fix for oneboxes in chat messages due to this case.

lib/onebox/engine/allowlisted_generic_onebox.rb

@@ -257,6 +257,11 @@ module Onebox
       end
 
       def article_html
+        if data[:image]
+          data[:thumbnail_width] ||= data[:image_width] || data[:width]
+          data[:thumbnail_height] ||= data[:image_height] || data[:height]
+        end
+
         layout.to_html
       end