Fixes #1559.

Problem

Reading Font.highlight_color on a run with <w:highlight w:val="none"/> raises ValueError: WD_COLOR_INDEX has no XML mapping for 'none'. none is a valid ST_HighlightColor value that Word writes when a highlight is explicitly cleared. Because the error fires during parsing, it aborts processing of any document containing the value.

from docx import Document
from docx.oxml.ns import qn

doc = Document()
r = doc.add_paragraph().add_run("hi")._r
hl = r.get_or_add_rPr().makeelement(qn("w:highlight"), {qn("w:val"): "none"})
r.get_or_add_rPr().append(hl)
doc.paragraphs[0].runs[0].font.highlight_color  # ValueError before this change

Fix

none means "not highlighted" — the same state python-docx already represents as None (returned when no w:highlight element is present, and what the highlight_val setter writes by removing the element). The getter now maps the explicit none value to None.

none is the only ST_HighlightColor value absent from WD_COLOR_INDEX, so this closes the gap without introducing an enum member that would collide on MS-API value 0 with AUTO.

Changes

• src/docx/oxml/text/font.py: CT_RPr.highlight_val returns None for w:val="none".
• tests/text/test_font.py: added a parametrized case asserting w:val=none reads as None.

Verification

• pytest — full suite (1610 passed), including the new case
• ruff check / ruff format --check — clean
• pyright on the changed source — 0 errors