[REG 6.8 -> 6.9] TypeError: Property '...' of object ... is not a function for slots created by QMetaObjectBuilder

When exposing a QObject created by QMetaObjectBuilder (as in PySide), slots added there no longer work, resulting in errors like:

 Property '...' of object ... is not a function for slots created by QMetaObjectBuilder()

Steps to reproduce:

• Apply https://codereview.qt-project.org/c/qt/qtdeclarative/+/597806
  to examples/qml/tutorials/extending-qml/chapter2-methods/CMakeLists.txt
• Run
• Click to clear the chart

  chapter2-methods
  Qt 6.9.0 (x86_64-little_endian-lp64 shared (dynamic) debug build; by GCC 13.2.0)
  getStaticMetaObject 66
  qmlRegisterTypeEx 0
  PieChart 66
  qt_metacall 1 19 0x7ffc2e0fd1a0
  qt_metacall Base -25
  qrc:/qt/qml/Charts/App.qml:21: TypeError: Property 'clearChart' of object
     PieChart(0x62e256a57880) is not a function
  

  chapter2-methods_68/chapter2-methods
  Qt 6.8.1 (x86_64-little_endian-lp64 shared (dynamic) debug build; by GCC 13.2.0)
  getStaticMetaObject 66
  qmlRegisterTypeEx 0
  PieChart 66
  qt_metacall 1 19 0x7ffd89b32eb0
  qt_metacall Base -25
  qt_metacall 0 66 0x7ffd89b30870
  doClearChart
  qml: The chart has been cleared
  

  qtbase at: 30d90b4ccad83ab1f23dab7cd72b7e228c299895
  qtdeclarative at: 99a72a051376101a163ba96a50ee20491347038a