fix: resolving issue 3655 for rich tables in docx

[PeterStaar-IBM]

Summary

Fix DOCX table extraction when tables are wrapped in block-level SDT content controls.

Some DOCX files store real w:tbl elements inside w:sdtContent. The MS Word backend previously handled block-
level w:sdt by extracting every nested paragraph directly, which preserved the table text but flattened the
table structure. This change recursively walks the direct w:sdtContent children so nested tables reach the
existing table parser.

Changes

• Recursively process block-level w:sdtContent in MsWordDocumentBackend._walk_linear.
• Add regression coverage for docx_rich_tables_01.docx.
• Verify the fixture extracts two 24x3 tables with headers:
  • Feature
  • Action Needed
  • Comment/Links
• Verify the tables appear after the Phase 1 and Phase 2 text markers.

resolves: #3655

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[ceberam]

Looks good.
@PeterStaar-IBM can you just add the groundtruth files for the new test file docx_rich_tables_01.docx ? They should be:

tests/data/groundtruth/docling_v2/docx_rich_tables_01.docx.itxt
tests/data/groundtruth/docling_v2/docx_rich_tables_01.docx.json
tests/data/groundtruth/docling_v2/docx_rich_tables_01.docx.md

[ceberam]

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