Fix hasTextHighlighting to check for "none" string

Since 1edf415, highlight-style: none sets "none" string instead of null.
The ?? operator and null check didn't catch this, causing hasTextHighlighting
to incorrectly return true when highlighting was disabled.

- Use || fallback (consistent with hasAdaptiveTheme, readHighlightingTheme)
- Check for "none" string instead of null
- Remove unreachable null check (|| chain always provides default)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cderv

src/quarto-core/text-highlighting.ts

@@ -98,8 +98,10 @@ export function hasAdaptiveTheme(pandoc: FormatPandoc) {
 
 export function hasTextHighlighting(pandoc: FormatPandoc): boolean {
   // Check both syntax-highlighting (new) and highlight-style (deprecated alias)
-  const theme = pandoc[kSyntaxHighlighting] ?? pandoc[kHighlightStyle];
-  return theme !== null;
+  const theme = pandoc[kSyntaxHighlighting] ||
+    pandoc[kHighlightStyle] ||
+    kDefaultHighlightStyle;
+  return theme !== "none";
 }
 
 export function isAdaptiveTheme(theme: string | Record<string, string>) {