fix: map highlight w:val="none" to None instead of raising

`w:val="none"` is a valid `ST_HighlightColor` value that Word writes when
a highlight is explicitly cleared from a run. It has no `WD_COLOR_INDEX`
member, so reading `Font.highlight_color` on such a run raised
`ValueError: WD_COLOR_INDEX has no XML mapping for 'none'` from
`BaseXmlEnum.from_xml()`, which aborts parsing of any document containing
the value.

`none` semantically means "not highlighted", which python-docx already
represents as `None` (the value returned when no `w:highlight` element is
present), and which the `highlight_val` setter writes by removing the
element. Map the explicit `none` value to `None` in the getter to match.

`none` is the only `ST_HighlightColor` value absent from `WD_COLOR_INDEX`,
so this closes the gap completely.

Author: mithunvoe

src/docx/oxml/text/font.py

@@ -8,7 +8,7 @@
 
 from docx.enum.dml import MSO_THEME_COLOR
 from docx.enum.text import WD_COLOR_INDEX, WD_UNDERLINE
-from docx.oxml.ns import nsdecls
+from docx.oxml.ns import nsdecls, qn
 from docx.oxml.parser import parse_xml
 from docx.oxml.simpletypes import (
     ST_HexColor,
@@ -159,6 +159,12 @@ def highlight_val(self) -> WD_COLOR_INDEX | None:
         highlight = self.highlight
         if highlight is None:
             return None
+        # -- `w:val="none"` is a valid `ST_HighlightColor` value meaning "no
+        # -- highlight". It has no `WD_COLOR_INDEX` member, so map it to `None`
+        # -- (the same value used when no `w:highlight` element is present)
+        # -- rather than raising on the unmapped value.
+        if highlight.get(qn("w:val")) == "none":
+            return None
         return highlight.val
 
     @highlight_val.setter

tests/text/test_font.py

@@ -381,6 +381,7 @@ def it_can_change_its_underline_type(
             ("w:r/w:rPr", None),
             ("w:r/w:rPr/w:highlight{w:val=default}", WD_COLOR.AUTO),
             ("w:r/w:rPr/w:highlight{w:val=blue}", WD_COLOR.BLUE),
+            ("w:r/w:rPr/w:highlight{w:val=none}", None),
         ],
     )
     def it_knows_its_highlight_color(self, r_cxml: str, expected_value: WD_COLOR | None):