fix: shift KV/Form graph cell page numbers during DoclingDocument.concatenate

[cau-git]

DoclingDocument.concatenate was updating DocItem.prov.page_no with page_delta, but it did not update GraphCell.prov.page_no inside KeyValueItem/FormItem graphs.
As a result, after concatenation, key-value/form graph cells from later documents could still point to page 1, causing incorrect visualization/page placement.

What changed

• In _DocIndex.index (docling_core/types/doc/document.py), when copying a KeyValueItem or FormItem, we now also shift cell.prov.page_no for every graph cell by page_delta.
• Added regression test test_concatenate_shifts_graph_cell_pages_for_keyvalue_and_form in test/test_docling_doc.py to verify concatenating two 1-page docs yields:
  • first KV/Form graph cells on page 1
  • second KV/Form graph cells on page 2

Why

This ensures graph-cell provenance remains consistent with concatenated page numbering, fixing KV/Form items appearing on the wrong page after merge.

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