Fix wrapping Object's in GDExtension that aren't exposed

[dsnopek]

This fixes a bug in PR #73511

The Object::get_class_name_for_extension() method is used to pick which wrapper class should be used to create the wrapper on the godot-cpp side. However, GDExtensions only have wrapper classes that are exposed, because those are the only ones listed in extension_api.json. For classes that aren't exposed, we should be creating the wrapper using the nearest parent class that is exposed.

For example, while working on an editor plugin in GDExtension, I was getting error spam in the console like:

ERROR: Cannot find instance binding callbacks for class 'EditorPropertyCheck'.
   at: get_instance_binding_callbacks (godot-cpp/src/core/class_db.cpp:320)
ERROR: Cannot find instance binding callbacks for class 'EditorPropertyVector2i'.
   at: get_instance_binding_callbacks (godot-cpp/src/core/class_db.cpp:320)
ERROR: Cannot find instance binding callbacks for class 'EditorPropertyResource'.
   at: get_instance_binding_callbacks (godot-cpp/src/core/class_db.cpp:320)
ERROR: Cannot find instance binding callbacks for class 'EditorPropertyText'.
   at: get_instance_binding_callbacks (godot-cpp/src/core/class_db.cpp:320)
ERROR: Cannot find instance binding callbacks for class 'EditorPropertyInteger'.
   at: get_instance_binding_callbacks (godot-cpp/src/core/class_db.cpp:320)
ERROR: Cannot find instance binding callbacks for class 'EditorPropertyFloat'.
   at: get_instance_binding_callbacks (godot-cpp/src/core/class_db.cpp:320)
ERROR: Cannot find instance binding callbacks for class 'EditorPropertyVector2'.
   at: get_instance_binding_callbacks (godot-cpp/src/core/class_db.cpp:320)

These are all child classes of EditorProperty which is exposed, so, on the GDExtension side, we want to create an EditorProperty wrapper for each of them.

This PR changes the Object::get_class_name_for_extension() method to check if the class is exposed, and if not, it walks up the classes parents until it finds one that is exposed.

[akien-mga]

Can't review in depth, but the overall changes seem fine to me.

[akien-mga]

Thanks!