fix: markdown export under -O

[Guiforge]

Fix Markdown export under Python optimized mode (-O)

Context

Issue: docling-project/docling-core#460

Markdown export could crash when running Python with optimizations enabled (e.g. python -O or PYTHONOPTIMIZE=1).

Symptom

UnboundLocalError: cannot access local variable 'list_group' where it is not associated with a value

This surfaced during Markdown serialization of list items.

Root cause

In docling_core/transforms/serializer/markdown.py (Markdown list item serialization), list_group was assigned via the walrus operator inside an assert:

• assert item.parent and isinstance((list_group := item.parent.resolve(doc)), ListGroup)

Under -O, Python strips assert statements entirely, so the assignment never executes. Since Python still treats list_group as a local variable in the function, later references to list_group raise UnboundLocalError.

(Conceptually the same as Ruff rule RUF018: assignment in assert.)

Fix

• Remove the assignment from the assert.
• Resolve list_group explicitly (outside of assert) and guard access with an is not None check.
• If the parent is unexpectedly missing or not a ListGroup, fall back to the safe Markdown marker - rather than crashing.

This preserves existing behavior for valid documents and prevents optimized-mode crashes.

Tests

• Added test/test_markdown_optimized_mode.py which spawns a subprocess with sys.executable -O and calls DoclingDocument.export_to_markdown() on a minimal document that triggers list marker normalization.

Notes

• No public APIs changed.
• Behavior is strictly more robust: malformed list structures no longer crash serialization in optimized mode.

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[cau-git]

@Guiforge thanks for this fix! The change looks good to me, but I would suggest to remove the test unit you provide, it should not require a test at this level.

Also, please follow the steps to remediate the DCO as posted by the github-bot here.

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[Guiforge]

I closed; it seems solved. 👍 (#456)