fix syntax highlighting: use AbstractHighlighter directly

QSyntaxHighlighter applies colors to the layout only (not accessible
via QTextBlock iteration). Replace with a custom AbstractHighlighter
subclass (HtmlBuilder) that builds HTML directly in applyFormat().
This actually produces colored output.

gui/chatblockmodel.cpp

@@ -4,8 +4,9 @@
 #include <KSyntaxHighlighting/Repository>
 #include <KSyntaxHighlighting/Definition>
 #include <KSyntaxHighlighting/Theme>
-#include <KSyntaxHighlighting/SyntaxHighlighter>
-#include <QTextDocument>
+#include <KSyntaxHighlighting/AbstractHighlighter>
+#include <KSyntaxHighlighting/State>
+#include <KSyntaxHighlighting/Format>
 #include <QGuiApplication>
 #include <QClipboard>
 #include <QJsonDocument>
@@ -329,6 +330,37 @@ QVariantList ChatBlockModel::parseSubBlocks(const QString &content) const
     return result;
 }
 
+// Custom highlighter that builds HTML string directly
+class HtmlBuilder : public KSyntaxHighlighting::AbstractHighlighter
+{
+public:
+    QString html;
+
+    using AbstractHighlighter::AbstractHighlighter;
+
+    KSyntaxHighlighting::State processLine(const QString &line, const KSyntaxHighlighting::State &state)
+    {
+        m_currentLine = line;
+        return highlightLine(line, state);
+    }
+
+protected:
+    void applyFormat(int offset, int length, const KSyntaxHighlighting::Format &format) override
+    {
+        QString text = m_currentLine.mid(offset, length).toHtmlEscaped();
+        if (format.hasTextColor(theme())) {
+            QColor fg = format.textColor(theme());
+            html += QStringLiteral("<span style=\"color:") + fg.name() + QStringLiteral(";\">")
+                  + text + QStringLiteral("</span>");
+        } else {
+            html += text;
+        }
+    }
+
+private:
+    QString m_currentLine;
+};
+
 QString ChatBlockModel::highlightedHtml(const QString &code, const QString &language) const
 {
     // Check cache first
@@ -339,55 +371,33 @@ QString ChatBlockModel::highlightedHtml(const QString &code, const QString &lang
 
     auto def = repo().definitionForName(language);
     if (!def.isValid()) {
-        // Try by file extension
         def = repo().definitionForFileName("file." + language);
     }
     if (!def.isValid()) {
-        // Return escaped plain text
-        QString plain = code.toHtmlEscaped().replace('\n', "<br>");
+        QString plain = code.toHtmlEscaped().replace('\n', QStringLiteral("<br>"));
         m_highlightCache.insert(cacheKey, plain);
         return plain;
     }
 
-    // Determine theme based on current palette
     bool dark = QGuiApplication::palette().color(QPalette::Window).lightnessF() < 0.5;
     auto theme = dark
         ? repo().defaultTheme(KSyntaxHighlighting::Repository::DarkTheme)
         : repo().defaultTheme(KSyntaxHighlighting::Repository::LightTheme);
 
-    QTextDocument doc;
-    doc.setPlainText(code);
+    HtmlBuilder builder;
+    builder.setDefinition(def);
+    builder.setTheme(theme);
 
-    auto *highlighter = new KSyntaxHighlighting::SyntaxHighlighter(&doc);
-    highlighter->setDefinition(def);
-    highlighter->setTheme(theme);
-    highlighter->rehighlight();
-
-    // Extract HTML from the highlighted document
-    QString html;
-    QTextBlock block = doc.begin();
-    while (block.isValid()) {
-        for (auto it = block.begin(); !it.atEnd(); ++it) {
-            QTextFragment fragment = it.fragment();
-            if (!fragment.isValid()) continue;
-
-            QTextCharFormat fmt = fragment.charFormat();
-            QString text = fragment.text().toHtmlEscaped();
-
-            if (fmt.foreground().style() != Qt::NoBrush) {
-                QColor fg = fmt.foreground().color();
-                html += QString("<span style=\"color:%1;\">%2</span>").arg(fg.name(), text);
-            } else {
-                html += text;
-            }
-        }
-        block = block.next();
-        if (block.isValid())
-            html += "<br>";
+    KSyntaxHighlighting::State state;
+    const QStringList lines = code.split('\n');
+    for (int i = 0; i < lines.size(); ++i) {
+        state = builder.processLine(lines[i], state);
+        if (i < lines.size() - 1)
+            builder.html += QStringLiteral("<br>");
     }
 
-    m_highlightCache.insert(cacheKey, html);
-    return html;
+    m_highlightCache.insert(cacheKey, builder.html);
+    return builder.html;
 }
 
 QString ChatBlockModel::prettyJson(const QString &text) const