Blockquoted dollar math equation results in error message

[e13h]

Describe the bug

When I have a blockquoted dollar math equation:

> $$
x = 2y
$$

I expect the formatter to treat this as valid markdown. Instead, I get the following error message:

$ mdformat test.md
Error: Could not format "test.md".

Formatted Markdown renders to different HTML than input Markdown. This is a bug
in mdformat or one of its installed plugins. Please retry without any plugins
installed. If this error persists, report an issue including the input Markdown
on https://github.com/hukkin/mdformat/issues. If not, report an issue on the
malfunctioning plugin's issue tracker.

This is a problem because blockquoted dollar math equations is valid in the myst markdown spec.

Reproduce the bug

Create a file test.md with the following contents:

> $$
x = 2y
$$

Run mdformat.

Expected no changes to the file, instead getting an error from the mdformat command (see above for error).

List your environment

python 3.10.12
ubuntu 22.04

mdformat==0.7.22
mdformat-myst==0.2.1

[e13h]

Dug into this a little more and discovered that the formatter inserts extra blockquotes > and myst renders these as actual greater-than signs in the equation, resulting in different HTML.

	Markdown	HTML
Input	> $$ x = 2y $$	<blockquote> <div class="math block"> x = 2y </div></blockquote>
Output	> $$ > x = 2y > $$	<blockquote> <div class="math block"> &gt; x = 2y &gt; </div></blockquote>

And oddly enough, if you add the blockquotes > to the subsequent lines in the dollar math equation, the formatter just adds more!

	Markdown	HTML
Input	> $$ > x = 2y > $$	<blockquote> <div class="math block"> &gt; x = 2y &gt; </div></blockquote>
Output	> $$ > > x = 2y > > $$	<blockquote> <div class="math block"> &gt; &gt; x = 2y &gt; &gt; </div></blockquote>

[e13h]

Related: executablebooks/mdit-py-plugins#125