`AssertionError` when parsing a page with raw HTML

Start from the traceback in markdown2.py: _hash_html_block_sub (the assert m is not None around line 927) called from _strict_tag_block_sub in _hash_html_blocks. Reproduce by feeding the pasted snippet to markdown() and confirm it fails on 2.5.5 but not 2.5.4; git log/bisect between those tags to find the regression commit. Done when the snippet parses with the HTML emitted unchanged and the existing test suite (e.g. tests/test_markdown2.py) still passes, ideally with a new regression test.

Describe the bug
AssertionError when parsing a page with raw HTML

  File "/Users/brass75/src/brassnet.biz/.venv/lib/python3.13/site-packages/render_engine_markdown/__init__.py", line 18, in parse
    return markdown(
        content,
        extras=extras.get("markdown_extras", []) if extras else [],
    )
  File "/Users/brass75/src/brassnet.biz/.venv/lib/python3.13/site-packages/markdown2.py", line 210, in markdown
    use_file_vars=use_file_vars, cli=cli).convert(text)
                                          ~~~~~~~^^^^^^
  File "/Users/brass75/src/brassnet.biz/.venv/lib/python3.13/site-packages/markdown2.py", line 504, in convert
    text = self._hash_html_blocks(text, raw=True)
  File "/Users/brass75/src/brassnet.biz/.venv/lib/python3.13/site-packages/markdown2.py", line 255, in inner
    text = func(md, text, *args, **kwargs)
  File "/Users/brass75/src/brassnet.biz/.venv/lib/python3.13/site-packages/markdown2.py", line 991, in _hash_html_blocks
    text = self._strict_tag_block_sub(text, self._block_tags_a, hash_html_block_sub)
  File "/Users/brass75/src/brassnet.biz/.venv/lib/python3.13/site-packages/markdown2.py", line 1120, in _strict_tag_block_sub
    block = callback(block.rstrip('\n'))  # remove trailing newline
  File "/Users/brass75/src/brassnet.biz/.venv/lib/python3.13/site-packages/markdown2.py", line 4417, in result
    return function(*args + rest, **combined)
  File "/Users/brass75/src/brassnet.biz/.venv/lib/python3.13/site-packages/markdown2.py", line 927, in _hash_html_block_sub
    assert m is not None
           ^^^^^^^^^^^^^
AssertionError

To Reproduce

Since you've read this far I can assume that you're planning on attending PyCon US
this year. If you are planning on attending and enjoy baseball you might be interested in
coming with some folks to see either a Dodgers game ahead of the conference or an Angels
game after the first sprint day.

<blockquote class="mastodon-embed" data-embed-url="https://twit.social/@brass75/115486728122010385/embed"
    style="background: #FCF8FF; border-radius: 8px; border: 1px solid #C9C4DA; margin: 0; max-width: 540px;
    min-width: 270px; overflow: hidden; padding: 0;"> <a href="https://twit.social/@brass75/115486728122010385"
        target="_blank" style="align-items: center; color: #1C1A25; display: flex; flex-direction: column;
        font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Oxygen, Ubuntu, Cantarell,
'Fira Sans', 'Droid Sans', 'Helvetica Neue', Roboto, sans-serif; font-size: 14px; justify-content: center;
letter-spacing: 0.25px; line-height: 20px; padding: 24px; text-decoration: none;">
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="32" height="32"
    viewBox="0 0 79 75"><path d="M63 45.3v-20c0-4.1-1-7.3-3.2-9.7-2.1-2.4-5-3.7-8.5-3.7-4.1 0-7.2 1.6-9.3
        4.7l-2 3.3-2-3.3c-2-3.1-5.1-4.7-9.2-4.7-3.5 0-6.4 1.3-8.6 3.7-2.1 2.4-3.1 5.6-3.1 9.7v20h8V25.9c0-4.1
        1.7-6.2 5.2-6.2 3.8 0 5.8 2.5 5.8 7.4V37.7H44V27.1c0-4.9 1.9-7.4 5.8-7.4 3.5 0 5.2 2.1 5.2
        6.2V45.3h8ZM74.7 16.6c.6 6 .1 15.7.1 17.3 0 .5-.1 4.8-.1 5.3-.7 11.5-8 16-15.6 17.5-.1 0-.2 0-.3 0-4.9
        1-10 1.2-14.9 1.4-1.2 0-2.4 0-3.6 0-4.8 0-9.7-.6-14.4-1.7-.1 0-.1 0-.1 0s-.1 0-.1 0 0 .1 0 .1 0 0 0
        0c.1 1.6.4 3.1 1 4.5.6 1.7 2.9 5.7 11.4 5.7 5 0 9.9-.6 14.8-1.7 0 0 0 0 0 0 .1 0 .1 0 .1 0 0 .1 0 .1
        0 .1.1 0 .1 0 .1.1v5.6s0 .1-.1.1c0 0 0 0 0 .1-1.6 1.1-3.7 1.7-5.6 2.3-.8.3-1.6.5-2.4.7-7.5 1.7-15.4
        1.3-22.7-1.2-6.8-2.4-13.8-8.2-15.5-15.2-.9-3.8-1.6-7.6-1.9-11.5-.6-5.8-.6-11.7-.8-17.5C3.9 24.5 4 20
        4.9 16 6.7 7.9 14.1 2.2 22.3 1c1.4-.2 4.1-1 16.5-1h.1C51.4 0 56.7.8 58.1 1c8.4 1.2 15.5 7.5 16.6 15.6Z"
        fill="currentColor"/></svg> <div style="color: #787588; margin-top: 16px;">
            Post by @[email protected]</div> <div style="font-weight: 500;">View on Mastodon</div> </a>
</blockquote> <script data-allowed-prefixes="https://twit.social/" async src="https://twit.social/embed.js"></script>

I hope to see you in Long Beach this May. And I hope you do take the plunge and submit
a talk.

Expected behavior
The included HTML is rendered unchanged.

Debug info
2.5.5 - this does not happen with 2.5.4

Any extras being used:
None.

Additional context