fix: render unrecognized HTML tags as plain text

[Soxasora]

Description

Context https://stacker.news/items/1371376
Adds a fallback visitor for HTML tags that creates a plain TextNode instead of going to the unrecognized markdown construct route and create a whole paragraph.

Screenshots

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After:

Checklist

Are your changes backward compatible? Please answer below:

For example, a change is not backward compatible if you removed a GraphQL field or dropped a database column.
Yes
On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:
8

For frontend changes: Tested on mobile, light and dark mode? Please answer below:
n/a

Did you introduce any new environment variables? If so, call them out explicitly here:
no

Did you use AI for this? If so, how much did it assist you?
no

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Note

Implements a plain-text fallback for mdast html nodes to avoid creating standalone paragraphs for unrecognized tags.

• Adds MdastHtmlFallbackVisitor that appends a TextNode with parent formatting for any html node
• Updates formattingVisitors order to run specific HTML tag visitors (sup, sub) before the fallback

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