fix: rows and columns with RichTableCell

[PeterStaar-IBM]

What

TableData.get_row_bounding_boxes / get_column_bounding_boxes only looked at cell.bbox. Since TableCell.bbox is
optional and a RichTableCell typically has bbox=None (its content lives behind ref), those cells were silently
skipped — so any row or column containing a rich cell got a bounding box that ignored the rich cell's actual
extent, or was omitted entirely.

How

Mirroring the existing _get_text pattern, this adds a _get_bbox method:

• TableCell._get_bbox(doc=None) returns self.bbox.
• RichTableCell._get_bbox(doc=None) override: an explicit bbox still wins; otherwise, when a doc is supplied, it
  resolves self.ref, walks the referenced subtree, and returns the union (BoundingBox.enclosing_bbox) of all
  descendant provenance boxes. Returns None when no doc / no provenance is available.

get_row_bounding_boxes / get_column_bounding_boxes gain an optional doc keyword and resolve each cell's bbox once
up front via _get_bbox(doc=doc). TableVisualizer forwards doc so the drawn row/column overlays pick up the
resolved boxes.

Compatibility

Backward compatible: doc defaults to None, so existing callers that pass no doc keep current (own-bbox-only)
behavior.

Tests

Adds test_table_data_bounding_boxes_with_rich_table_cell, which builds a table mixing a RichTableCell (bbox=None,
ref → group of text items with prov boxes) and a plain cell, and asserts:

• _get_bbox returns None without a doc and the correct union with one;
• without doc the rich cell is skipped (row covers only the plain cell, rich-only column absent);
• with doc the row/column boxes enclose the referenced extent;
• an explicit bbox on the rich cell takes precedence.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling_core/types/doc/document.py	93.75%	2 Missing ⚠️

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[github-actions]

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