fix(docx): preserve parent state when processing table cells

[Br1an67]

Issue resolved by this Pull Request:
Resolves #2668

Summary

Fixed a bug where section headers appearing after tables in DOCX documents were incorrectly being added as children of table header cells. This caused the document hierarchy to be corrupted, resulting in malformed markdown and other exports.

Root Cause

The _handle_tables method in msword_backend.py calls _walk_linear to process rich table cell content. However, _walk_linear modifies self.parents (especially when cells contain headings). This parent state was never restored after processing each cell, polluting the parent hierarchy for subsequent document elements.

Changes

• Save self.parents.copy() before calling _walk_linear in two places within _handle_tables:
  1. For rich table cells (multi-cell tables)
  2. For 1x1 tables treated as furniture
• Restore self.parents after processing each cell

This follows the same pattern already used in _handle_textbox_content.

Testing

• All existing msword backend tests pass
• Verified fix with a test case that creates a table followed by a section header - the header now correctly appears at the document level, not nested within the table

Checklist:

• Documentation has been updated, if necessary. (Not necessary - internal fix)
• Examples have been added, if necessary. (Not necessary)
• Tests have been added, if necessary. (Existing tests cover this area)

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[Br1an67]

Closing this in favor of #3047, which includes more comprehensive state restoration (saves/restores self.parents, self.level, and self.level_at_new_list) and has test coverage. I'll incorporate the 1x1 furniture table fix from this PR into #3047.