WebVTT inline formatting emits Markdown whose emphasis does not render #4352

Bug

WebVTT cue text that uses inline markup produces Markdown whose emphasis does not render. The span's boundary whitespace stays inside the formatted text item, and the inline-group serializer adds its own separator on top, so the output gets both a doubled space and, where spans are nested, an asterisk with a space inside it.

A closing * preceded by whitespace is not a right-flanking delimiter in CommonMark, so it cannot close emphasis. The markup is emitted but silently fails to render.

This is visible in the repository's own checked-in ground truth, tests/data/webvtt/groundtruth/webvtt_example_04.vtt.md:

The waiter offered a  *steaming bowl of * *paella*  that instantly transported the diners to a sunny Mediterranean coast.
The dessert's  ***unexpected*** * * *arcobaleno* * of flavors*  left everyone in awe.

Rendered through a CommonMark renderer, the italics are lost and the asterisks show up as literal text:

<p>The waiter offered a  *steaming bowl of * <em>paella</em>  that arrived.</p>
<p>The dessert's  <em><strong>unexpected</strong></em> * * <em>arcobaleno</em> * of flavors*  left everyone in awe.</p>

Steps to reproduce

import io
from docling.document_converter import DocumentConverter
from docling.datamodel.base_models import InputFormat, DocumentStream

vtt = (
    "WEBVTT\n\n00:00:01.000 --> 00:00:04.000\n"
    "The waiter offered a <i>steaming bowl of paella</i> that arrived.\n"
)

converter = DocumentConverter(allowed_formats=[InputFormat.VTT])
result = converter.convert(DocumentStream(name="t.vtt", stream=io.BytesIO(vtt.encode())))
print(repr(result.document.export_to_markdown()))

Actual:

'The waiter offered a  *steaming bowl of paella*  that arrived.'

Expected:

'The waiter offered a *steaming bowl of paella* that arrived.'

The single-span case above only produces the doubled space. Nesting a span inside another, as webvtt_example_04.vtt does with <i>steaming bowl of <lang es-ES>paella</lang></i>, is what moves a space inside the emphasis markers and breaks the rendering.

Notes

This looks like the same class of problem as #2745 (docx adds an extra space after bold characters) and #4314. PR #3891 reworks the InlineGroup whitespace contract for the html, md, msword, opendocument and jats backends, but it does not touch webvtt_backend.py, so this backend would still be affected after it lands. Flagging it so it can be picked up with that work rather than fixed in a way that collides with it.

Docling version

2.129.0 (main @ 890dd42)

Python version

3.12.13