feat(msword): parse native Word charts as classified pictures with data

[DanielNg0729]

Hi maintainers, This PR follow up with #3777 and #3794 which adds Word parity for the native-chart handling already shipped for Excel and PowerPoint. Charts embedded in .docx files (a w:drawing graphic frame referencing word/charts/chartN.xml) are now parsed into classified PictureItems that carry the chart's underlying data reconstructed as a table.

What changed

• Native charts are detected during the document walk and emitted as a PictureItem whose meta carries:
  • a chart-type classification (bar_chart, line_chart, pie_chart, scatter_chart, other_chart) derived from the plot element under c:plotArea;
  • the chart's plotted numbers reconstructed as a TabularChartMetaField, read directly from the inline c:numCache/c:strCache — no LibreOffice required, fully deterministic.
• New backend option MsWordBackendOptions.render_chart_images (default False). When enabled (and LibreOffice is available), a rendered image is attached to the chart picture via the existing DOCX→PDF→PNG path; charts keep their classification and data regardless.
• Non-chart DrawingML (shapes, SmartArt) is unchanged and still rasterized via LibreOffice.

Behavior change

Previously a chart was rendered as a generic, unclassified image by default whenever LibreOffice was present (and produced nothing without it). It is now a classified picture with tabular data by default, and the rendered image is opt-in via render_chart_images=True. This matches the Excel/PPTX chart behavior. The rendered image is LibreOffice's rendering, not a pixel copy of Word's styling.

Tests & reference data

• Reuses the existing drawingml.docx fixture (a line chart with 3 series).
• Adds tests for: classification + data reconstruction without LibreOffice; the opt-in render path; and the opt-out (no image by default) — the last two gated on a LibreOffice install.
• Regenerated drawingml.docx groundtruth: the chart picture now carries its classification + data table (and no default image); other pictures unchanged.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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Codecov Report

❌ Patch coverage is 85.78947% with 27 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/backend/msword_backend.py	85.48%	27 Missing ⚠️

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