Return wxACC_NOT_IMPLEMENTED from wxWindowAccessible::GetParent()

If a widget has a custom accessibility implementation derived from
wxWindowAccessible, there's no guarantee that the parent overrides the
nullptr-returning base CreateAccessible.

Returning wxACC_NOT_IMPLEMENTED follows the pattern of other
accessibility methods to defer to the platform's default implementation,
and fixes wx wxWindowAccessible::GetParent() when called on custom
accessibility implementations like wxCheckListBoxAccessible.

Closes #26063.

src/common/wincmn.cpp

@@ -3955,7 +3955,7 @@ wxAccStatus wxWindowAccessible::GetParent(wxAccessible** parent)
         if (*parent)
             return wxACC_OK;
         else
-            return wxACC_FAIL;
+            return wxACC_NOT_IMPLEMENTED;
     }
 }