Fix {module} placeholder resolution in module bay position field (#21752)

* Fix {module} placeholder resolution in module bay position field (#20467)

The {module} placeholder in ModuleBayTemplate's position field was not
being resolved when a module was installed, leaving the literal string
"{module}" in the position. This adds a resolve_position() method and
calls it in instantiate(), consistent with how resolve_name() and
resolve_label() already work.

Consolidates the shared resolution logic into _resolve_module_placeholder()
to eliminate duplication across resolve_name, resolve_label, and the new
resolve_position.

Fixes: #20467

* Move resolve_position() to ModuleBayTemplate

---------

Co-authored-by: Jeremy Stretch <jstretch@netboxlabs.com>

netbox/dcim/tests/test_models.py

@@ -849,6 +849,50 @@ class ModuleBayTestCase(TestCase):
         nested_bay = module.modulebays.get(name='SFP A-21')
         self.assertEqual(nested_bay.label, 'A-21')
 
+    @tag('regression')  # #20467
+    def test_nested_module_bay_position_resolution(self):
+        """Test that {module} in a module bay template's position field is resolved when the module is installed."""
+        manufacturer = Manufacturer.objects.first()
+        site = Site.objects.first()
+        device_role = DeviceRole.objects.first()
+
+        device_type = DeviceType.objects.create(
+            manufacturer=manufacturer,
+            model='Device with Position Test',
+            slug='device-with-position-test'
+        )
+        ModuleBayTemplate.objects.create(
+            device_type=device_type,
+            name='Slot 1',
+            position='1'
+        )
+
+        module_type = ModuleType.objects.create(
+            manufacturer=manufacturer,
+            model='Module with Position Placeholder'
+        )
+        ModuleBayTemplate.objects.create(
+            module_type=module_type,
+            name='Sub-bay {module}-1',
+            position='{module}-1'
+        )
+
+        device = Device.objects.create(
+            name='Position Test Device',
+            device_type=device_type,
+            role=device_role,
+            site=site
+        )
+        module_bay = device.modulebays.get(name='Slot 1')
+        module = Module.objects.create(
+            device=device,
+            module_bay=module_bay,
+            module_type=module_type
+        )
+
+        nested_bay = module.modulebays.get(name='Sub-bay 1-1')
+        self.assertEqual(nested_bay.position, '1-1')
+
     @tag('regression')  # #20912
     def test_module_bay_parent_cleared_when_module_removed(self):
         """Test that the parent field is properly cleared when a module bay's module assignment is removed"""