Fix typing of approximate diameter and add typing to function for it

Seems I forgot to add typing and that's biting my bum right now.

Contributes to issue CURA-6600.

cura/Machines/Models/MaterialManagementModel.py

@@ -151,7 +151,7 @@ class MaterialManagementModel(QObject):
         # Find the preferred material.
         extruder_stack = application.getMachineManager().activeStack
         active_variant_name = extruder_stack.variant.getName()
-        approximate_diameter = str(extruder_stack.approximateMaterialDiameter)
+        approximate_diameter = int(extruder_stack.approximateMaterialDiameter)
         machine_node = ContainerTree.getInstance().machines[application.getGlobalContainerStack().definition.getId()]
         preferred_material_node = machine_node.variants[active_variant_name].preferredMaterial(approximate_diameter)
 

cura/Machines/VariantNode.py

@@ -79,7 +79,7 @@ class VariantNode(ContainerNode):
     #   material.
     #   \return The node for the preferred material, or any arbitrary material
     #   if there is no match.
-    def preferredMaterial(self, approximate_diameter) -> MaterialNode:
+    def preferredMaterial(self, approximate_diameter: int) -> MaterialNode:
         for base_material, material_node in self.materials.items():
             if self.machine.preferred_material in base_material and approximate_diameter == int(material_node.getMetaDataEntry("approximate_diameter")):
                 return material_node