fix: return None for w:highlight w:val="none" instead of raising ValueError

[VenkateswarluNagineni]

Summary

Fixes #1559.

Reading Font.highlight_color on a run whose <w:highlight> element has w:val="none" raised ValueError: WD_COLOR_INDEX has no XML mapping for 'none'. This crashed processing of the entire document even though none is a valid ST_HighlightColor value that Word writes when explicitly clearing a highlight from a run.

Root cause

CT_RPr.highlight_val forwarded highlight.val directly to the RequiredAttribute("w:val", WD_COLOR_INDEX) descriptor, which calls WD_COLOR_INDEX.from_xml("none"). Since "none" is absent from WD_COLOR_INDEX (it has no distinct MS-API integer), from_xml raised.

Fix

Check the raw w:val attribute before the enum lookup. w:val="none" means "not highlighted" — semantically identical to the no-element case — so return None:

# w:val="none" is a valid ST_HighlightColor value meaning "no highlight";
# it's not in WD_COLOR_INDEX so we return None to match the no-element case.
if highlight.get(qn("w:val")) == "none":
    return None

Testing

Added a parametrized test case ("w:r/w:rPr/w:highlight{w:val=none}", None) to it_knows_its_highlight_color. All 11 highlight-related tests pass; no regressions.

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[This is Claude Code on behalf of Venkateswarlu Nagineni]