Fix cell backgrounds for self-closing tcPr tags

[ahmetmusab42-stack]

Summary

• normalize a self-closing <w:tcPr /> before inserting cell shading
• keep the generated <w:shd> inside <w:tcPr> so Word preserves the background
• add a focused regression test for shorthand table-cell properties

Verification

• uv run python tests/cellbg_shorthand.py
• uv run flake8 docxtpl tests setup.py --count --max-line-length=127 --show-source --statistics
• rendered the document4.docx attachment from Cell background tags do not work in docx files with shorthand tags (i.e., with <w:tcPr /> instead of <w:tcPr>...</w:tcPr>) #559 and verified word/document.xml contains <w:tcPr><w:shd .../></w:tcPr> with no direct <w:tc>/<w:shd> child
• uv build

Fixes #559

[ahmetmusab42-stack]

I missed #656 when checking for overlapping work. It already fixes #559 more comprehensively by covering both cellbg and colspan, with broader regression coverage. Closing this as a duplicate rather than asking maintainers to review redundant work. Sorry for the noise.