[pandasuwu]

The Markdown list serializer indents only nested lists. Other children of a ListItem (code, tables, formulas, pictures, text) are written at the indentation of the item marker. A CommonMark parser then ends the item before them. When the item has no text, the parser reads an empty item, and the next item starts a new list with its own numbering.

Example from the LaTeX backend (docling-project/docling#4518):

\begin{enumerate}
\item \begin{verbatim}code\end{verbatim}
\item Second
\end{enumerate}

Before:

1. 
```
code
```
2. Second

A CommonMark parser reads an empty item 1., a code block outside the list, and a new list that starts at 2..

After:

1. 
   ```
   code
   ```
2. Second

Fix. Each line of a non-list child of a list item is indented to the content of that item, that is, by the width of the item marker: 2 for -, 3 for 1., 4 for 10.. Blank lines are not indented, so a code block keeps its blank lines. Nested lists keep their own indentation, as before.

Tests. Four new tests in tests/test_serialization.py, all failing on main:

• test_md_list_item_block_children_are_indented: an empty item with a code block (with a blank line) and text, and an item with text and a formula.
• test_md_list_item_block_children_follow_marker_width: a code block under - and under 10..
• test_md_nested_list_item_block_children_are_indented: a code block in a nested item, and one after the nested list.
• test_md_list_item_table_with_caption_outside_list_is_indented: a table in an item whose caption is not in the list, so the serialized table starts with an item from outside the list.

Regenerated with DOCLING_GEN_TEST_DATA=1: no Markdown ground truth changes.

Downstream. I regenerated the docling ground truth of the non-ML backends with this branch. Compared with docling-core main, only these change: a picture inside a list item is now indented into its item in asciidoc_03.asciidoc.md, html_nested_block_in_list_item.html.md and wiki_duck.html.md, and one continuation line of a list item in the LaTeX 2305.03393_main.tex.md.

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