Fix flattening of visually-nested DOCX lists that use a separate numbering definition

[changsheng (changsheng0804-blip)]

Description

Fixes #2323.

When a Word document contains a visually-nested sub-list such as

1. Item 1
2. Item 2
   • Item 2.1
   • Item 2.2
3. Item 3
   a) Item 3.1
   b) Item 3.2

the current output flattens the a) / b) sub-list into the top level and renumbers its items (4., 5.).

Root cause: Word frequently stores such sub-lists as an independent top-level numbering definition (their own numId at ilvl 0); the visual nesting comes only from the level's left indent in numbering.xml. Since mammoth resolves nesting from ilvl, it emits the sub-items as siblings of the parent list, and the Markdown conversion faithfully flattens them. (Verified by inspecting the intermediate mammoth HTML and the document/numbering XML of the repro file from the issue: the sub-list paragraphs reference numId=3, ilvl=0 with left indent 1080, while the parent list level uses 720.)

Fix: a new _pre_process_nested_lists step in pre_process_docx rewrites such runs to continue the parent list at a deeper ilvl. A run is rewritten only when all of the following hold, keeping the heuristic conservative:

• it starts at ilvl 0 with a numId different from the immediately preceding numbered paragraph (any non-list paragraph in between prevents the rewrite);
• the indent of the new list's level 0 (from numbering.xml, w:left/w:start) is strictly greater than the indent of the preceding paragraph's list level;
• the referenced numbering definitions exist.

Documents without a numbering.xml part, and any file in which preprocessing raises, are passed through unchanged (same fallback behavior as the existing math preprocessing step).

Test evidence

All run locally (Windows, Python 3.12 venv, pip install -e packages/markitdown[docx]):

• The repro DOCX attached to Nested list in docx is not properly parsed in Markdown #2323 converts from the flattened form (4. Item 3.1 / 5. Item 3.2 at top level) to a properly nested list after this change.
• Three new tests in packages/markitdown/tests/test_docx_nested_lists.py (self-contained; they build minimal DOCX files in memory instead of adding binary fixtures):
  • the Nested list in docx is not properly parsed in Markdown #2323 pattern converts to a nested list;
  • consecutive lists at the same indent keep their own numbering (no false rewrite);
  • lists separated by a plain paragraph are not merged (no false rewrite).
• Existing vector tests: the failure set is byte-identical with and without this change (the failures in my minimal environment are missing optional converter dependencies, e.g. [xlsx]/[pptx], unrelated to DOCX); the test.docx conversion vector passes.
• black --check (repo-pinned 23.7.0) passes on both changed files.

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