fix: consider rich table cell refs in tree operations

[vagenas]

Fix: Consider rich table cell refs during tree operations

This commit fixes an issue where tree operations weren't handling references from RichTableCell objects, causing broken document hierarchies.

Changes:

1. Updated _shift_up: When removing a node, updates any table cell references to point to the node's child instead (errors if the node has multiple or no children).
2. Updated _shift_down: Updates table cell references to point to the new wrapper node before moving the subtree.
3. Updated _move_subtree: Prevents moving nodes that are referenced by table cells.
4. Improved fix_misplaced_list_items auto-healing: Uses updated _shift_down and _move_subtree instead of deleting and recreating items, which preserves references correctly.

Tests:

• Added 3 test cases for rich table cell reference handling
• Updated expected output for misplaced list items test

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[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling_core/types/doc/document.py	89.47%	2 Missing ⚠️

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