test(docx): add issue 176 regression fixture

[developer0hye]

What changed

Adds the public issue #176 DOCX attachment as a regression fixture so the drawing/list/code/table scenario keeps running in CI.

Key checks added:

• conversion smoke test for the fixture
• parsed IR contains the two rectangle shapes and connector arrow
• floating text boxes keep their text
• list numbering continues and nested items stay nested
• final table remains four rows by two columns
• code block hard line break and syntax-highlight character style are preserved

Verification

• cargo test -p office2pdf --test docx_fixtures issue_176 -- --nocapture
• cargo test -p office2pdf --test docx_fixtures
• cargo test -p office2pdf

Related: #176