[WIP] Add check for HTML void tags in epubs

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>epubs incomplete when viewed using Adobe Digital editions</issue_title>
<issue_description>Thanks for creating this repository. I just tried viewing the current epub versions of The Codex and Replacing guilt and found both to be incomplete. For example, the first section of The Codex ("Eight Short Studies on Excuses) has a subtitle ("The clumsy game player") but nothing else.</issue_description>

<agent_instructions>Add a check for html void tags on incoming content and automatically insert a close to the tag (e.g. insert </img> for img tags, </br> for br tags, '` for hr tags, etc so the output is valid XML instead of HTML5.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@AABoyles Hmmm. Sorry I'm six months late, but I'll look into this. @AABoyles Current hypothesis is that this is effectively the same bug as #6, which I believe is a disagreement between the HTML inputs and the XML schema that EPUBs require. Working on it...

• Fixes epubs incomplete when viewed using Adobe Digital editions #6

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