feat(serializer): add MsExcelMarkdownDocSerializer for sheet-name headings

[Smeet23]

Summary

• Adds MsExcelMarkdownFallbackSerializer and MsExcelMarkdownDocSerializer to docling_core/transforms/serializer/markdown_excel.py
• These classes enable rendering GroupLabel.SHEET groups as level-2 Markdown headings (## <sheet_name>) when exporting Excel-sourced DoclingDocuments
• Requested in feat(xlsx): render sheet names as headings in markdown export docling#3255 — the serializer was originally placed in the docling repo but belongs here per reviewer feedback

Test plan

• Verify MsExcelMarkdownDocSerializer renders sheet names as ## <name> headings
• Verify non-SHEET groups are handled by the standard fallback logic
• Verify the import path from docling_core.transforms.serializer.markdown_excel import MsExcelMarkdownDocSerializer works correctly

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[mergify]

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• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[github-actions]

✅ DCO Check Passed

Thanks @Smeet23, all your commits are properly signed off. 🎉

[PeterStaar-IBM]

lgtm!

[PeterStaar-IBM]

@Smeet23 Please run the DCO and make sure all checks work!

[Smeet23]

@PeterStaar-IBM Done! Added a DCO remediation commit and all checks are now passing — DCO, dco_advisor, and Mergify merge protections are all green. Thanks for the review and the approval!

[PeterStaar-IBM]

@Smeet23 Please run the uv run pre-commit run --all-files

[Smeet23]

Hi @PeterStaar-IBM — thanks for the earlier LGTM! I've pushed a small follow-up fix: the ruff formatter flagged one line in markdown_excel.py as too long and reformatted it — that was the only reason CI was failing. Everything is now passing (DCO ✅, Mergify ✅). Could you take a quick look at the new commit and re-approve when you get a chance? Happy to address any other concerns. Thanks!

[codecov]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...cling_core/transforms/serializer/markdown_excel.py	0.00%	19 Missing ⚠️

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