fix(xlsx): preserve headers after section labels

[Success6666]

Summary

• split a leading merged section label out of an XLSX table when it is directly attached to the real header row
• keep the real column header row as table headers after the split
• add regression coverage for the generated DoclingDocument, exported HTML, and section-label split helper boundaries

Closes #3687

Tests

• uv run --python 3.12 --extra convert-core --extra format-office --extra format-pdf --extra format-latex --extra models-local pytest tests\test_backend_msexcel.py::test_table_with_title tests\test_backend_msexcel.py::test_edge_cases_merging tests\test_backend_msexcel.py::test_gap_tolerance_comparison tests\test_backend_msexcel.py::test_merged_section_label_above_table_preserves_column_headers tests\test_backend_msexcel.py::test_split_leading_section_label_helper -q
• uv run --python 3.12 --extra convert-core --extra format-office --extra format-pdf --extra format-latex --extra models-local pytest tests\test_backend_msexcel.py::test_table_with_title tests\test_backend_msexcel.py::test_edge_cases_merging tests\test_backend_msexcel.py::test_gap_tolerance_comparison tests\test_backend_msexcel.py::test_merged_section_label_above_table_preserves_column_headers tests\test_backend_msexcel.py::test_split_leading_section_label_helper -q --cov=docling.backend.msexcel_backend --cov-report=term-missing
• uv run --python 3.12 ruff check docling\backend\msexcel_backend.py tests\test_backend_msexcel.py
• uv run --python 3.12 ruff format --check docling\backend\msexcel_backend.py tests\test_backend_msexcel.py
• git diff --check -- docling/backend/msexcel_backend.py tests/test_backend_msexcel.py

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Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/backend/msexcel_backend.py	90.00%	2 Missing ⚠️

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[Copilot]

Pull request overview

This PR updates the Excel backend to split out a leading merged “section label” row that is directly attached to a real table header row, so the true column headers remain headers (both in the DoclingDocument table model and in exported HTML). It also adds regression tests to cover the reported issue scenario.

Changes:

• Split a leading merged section label out of detected Excel tables and emit it as a separate TextItem.
• Preserve the real column header row as column_header=True after the split (so it renders as <th> in HTML).
• Add regression tests covering both the DoclingDocument structure and HTML output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
docling/backend/msexcel_backend.py	Adds _split_leading_section_label() and integrates it into table detection to extract section labels as text and preserve column headers.
tests/test_backend_msexcel.py	Adds regression tests verifying the section label is not absorbed into the table header and that HTML uses <th> for real headers.

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