Book.Description is publisher_summary verbatim, which is HTML. Audible's .aaxc
files embed an already-flattened copy, so the fallback only fired on Widevine
downloads - whose DASH files carry no description tags at all - and wrote raw
<p> markup into the file's comment and long-description atoms.
Move the grid's existing HtmlAgilityPack flattening into a shared helper that
also de-entitizes, and use it for the tag fallback. Extract the tag-filling body
of the RetrievedMetadata handler so it can be exercised directly by a test.
Co-authored-by: rmcrackan <rmcrackan@gmail.com>
