fix: restore visited set when TripletTableSerializer produces empty output

[OfekDanny]

Problem

Closes #3335 (reported against docling, root cause is in docling-core).

DOCX files that use layout tables for formatting produce zero chunks from both HierarchicalChunker and HybridChunker, even though doc.export_to_text() returns substantial content.

Root cause

Layout tables are represented as TableItem objects whose cells are RichTableCell entries pointing to TextItem children of the table. During chunking:

1. HierarchicalChunker calls doc_serializer.serialize(item=table, visited=visited).
2. DocSerializer.serialize() marks the TableItem itself in visited, then calls TripletTableSerializer.serialize().
3. TripletTableSerializer calls _export_to_dataframe_with_options(), which internally calls doc_serializer.serialize() for every RichTableCell reference, marking all child items in the shared visited set.
4. If the layout table serializes to empty text (e.g. all rows are column headers so there are no data rows for the triplet format), every child TextItem is permanently visited.
5. The chunker's main loop skips all visited items → 0 chunks.

The workaround in the issue (labels.discard(DocItemLabel.TABLE)) bypasses the table entirely, avoiding visited-set poisoning, but is not a correct general fix.

Fix

In TripletTableSerializer.serialize(), snapshot visited before calling _export_to_dataframe_with_options. If the table produces no output text, restore visited to the snapshot so child items remain available for chunking.

visited_before: set[str] = set(visited) if visited is not None else set()

# ... export & render the table ...

if not text_res and visited is not None:
    visited.clear()
    visited.update(visited_before)

This is safe: tables that produce text continue to work exactly as before. Only tables that produce no output have their visited side-effects undone.

Test

Added test_layout_table_produces_chunks to test/test_hierarchical_chunker.py. It builds a minimal two-row, one-column table where both rows are column headers (so TripletTableSerializer produces empty text), and asserts that HierarchicalChunker still yields one chunk per TextItem child.

All existing chunker tests continue to pass.

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