Session 025: Fix QTextCodec migration issues

- Z/src/qsql_informix/qsql_informix.cpp: Fixed QTextCodec include path
  (QtCore → QtGui) as QTextCodec moved to QtGui module in Qt6
- Z/src/qsql_informix/CMakeLists.txt: Added Qt6::Gui dependency required
  for QTextCodec
- Lts/Tools/TextEditorTest/textedit.cpp: Fixed Qt::codecForHtml →
  QTextCodec::codecForHtml (static method on QTextCodec class in Qt6)

These are minimal fixes required for Qt6 compatibility. QTextCodec is now
part of QtGui module and codecForHtml is a static method on QTextCodec
class rather than Qt namespace function.

Co-authored-by: Copilot <2235566219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/Lts/Tools/TextEditorTest/textedit.cpp

@@ -367,7 +367,7 @@ bool TextEdit::load(const QString &f)
     return false;
 
   QByteArray data = file.readAll();
-  QTextCodec *codec = Qt::codecForHtml(data);
+  QTextCodec *codec = QTextCodec::codecForHtml(data);
   QString str = codec->toUnicode(data);
   if (Qt::mightBeRichText(str)) {
     QUrl baseUrl = (f.front() == QLatin1Char(':') ? QUrl(f) : QUrl::fromLocalFile(f)).adjusted(QUrl::RemoveFilename);