[inchang-ing]

Description

Fixes #1096.

A raw HTML block whose start tag is indented by more than three spaces and sits inside a containing block (a list item, an admonition, etc.) was not extracted by the HTML block preprocessor, because at that point the indentation cannot be told apart from an indented code block. The block parser then treated the HTML as inline content and split it into separate paragraphs at every blank line.

This takes the "container" approach prototyped by @facelessuser in facelessuser/markdown (branch html-indent). Once a container has removed its own indentation, a block which starts with a block level tag can only be a raw block, so a new RawHtmlBlockProcessor extracts it there using the same extractor as the preprocessor, or the md_in_html extractor when that extension is enabled. Containers pass their content on a block at a time, so a raw block containing blank lines is collected across calls, keyed by the container, until it is closed, and RawHtmlFlushTreeprocessor finishes anything still open once the document has been parsed. The preprocessor is not modified, so extensions which work on the document text before it is parsed see exactly what they saw before. Content of a blockquote is deliberately left alone to keep its current behavior.

The two implementation commits are @facelessuser's, included unmodified and with their original authorship: Extract raw HTML blocks nested in containers in a block processor and Handle elements under blockquotes and fix comments. The third commit only adds the changelog entry. The approach is considerably simpler than the treeprocessor-based one in #1666, which this PR supersedes.

Verification

python -m unittest discover tests passes: 1310 tests, 111 skipped, 0 failures. flake8 reports nothing on the touched source files.

The behavior was also checked by rendering the same three inputs against the parent commit and against this branch. A raw block with a blank line inside a list item was rendered as <p><div> / <p>second paragraphs before and as a single <div> after; the same input inside a blockquote likewise; a four-space indented code block containing the same markup is unchanged in both, still escaped inside <pre><code>.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.

• AI tools were used and I have disclosed which ones, and have fully reviewed and verified their output.

  AI tools were use to assist with the following:

  • Code
  • Code Comments
  • Documentation
  • Changelog Entry
  • Commit Message
  • The Description Above
  • Other: ______________

  Claude was used to write the PR description, the changelog entry, and the message of the changelog commit. The implementation commits were authored by @facelessuser in his fork; he disclosed in Fix HTML blocks nested in indented blocks #1666 that he used AI to compile the list of cases that should pass while prototyping. I ran the test suite and the before/after renders myself and can explain any part of the change.

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the Documentation Style Guide.
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.