fix: html serialization of lists with mixed markers

[julianfesel]

This PR implements option 1 of issue #699

HTML list items with an empty marker now render with list-style-type: none; when original markers are being shown, so the browser no longer invents numbering that is not present in the DoclingDocument.

Changes

• Updated HTMLTextSerializer to suppress default list markers for unmarked list items while preserving explicit markers.
• Added a regression test for a mixed enumerated/non-enumerated list group.
• Refreshed the affected HTML and chunker goldens to match the new serializer output.

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[dolfim-ibm]

@julianfesel are all the changes produced on the test files what you would expect?

[julianfesel]

@julianfesel are all the changes produced on the test files what you would expect?

@dolfim-ibm

For show_original_list_item_marker=True, yes. However, based on your comment, I realized that I had misunderstood the role of marker: "" in Docling. I did not realize that it is not nullable.

In the new commits, I narrowed the application of strict markers to enumerated lists that contain at least one detected marker. As a result, there is now only one change in the test cases, which I believe is correct.

In my opinion, automatic enumeration does not make sense when at least one numeric marker is present. At the very least, this would require a more detailed validation. In such cases, one cannot assume that the default enumeration is correct. Furthermore, especially in OCR scenarios, the marker may actually be part of the text itself.

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