[Chen Yufeiyang (feiiiiii5)]

Fixes #2806.

DOCX math: a radical whose degree is hidden (m:radPr/m:degHide) is rendered with the degree anyway. do_rad only checked that the m:deg text was non-empty, and the Pr helper never parsed degHide because radPr children outside tag2meth were skipped.

This adds an on/off property mapping to Pr (an absent m:val or 1/true/on means on; 0/false/off means off) and makes do_rad fall back to the default radical when the degree is hidden. Visible degrees, empty radPr and absent radPr keep their current output.

Tested with parameterized cases for all seven shapes from the issue (hidden val=1/true/absent and visible val=0/false, empty radPr, no radPr):

# parent revision: 3 failed, 4 passed - assert "\sqrt[3]{x}" == "\sqrt{x}"
# this branch:     7 passed
# test_docx.py:   108 passed
# end-to-end: MarkItDown.convert_stream on a synthetic DOCX with degHide emits $\sqrt{x}$
# black --check:  two files unchanged

Not covered: a real Word document round trip (the DOCX is synthesized with python-docx), the wider [all] test matrix (optional dependencies are not installed here), and non-DOCX formats.