fix: File Upload widget disabled prop treats undefined as true (#5528)

* fix file upload widget disabled prop

* [test] extract createMockWidget to shared test utility - addresses @DrJKL's code replication concern

Creates testUtils.ts with shared createMockWidget and createMockFile functions
to reduce duplication across widget component tests. This ensures consistency
and maintainability of test setup code.

* [test] replace type assertions with type narrowing - addresses @DrJKL's type safety suggestion

Replaces unsafe `as HTMLInputElement` casts with proper instanceof checks
and error throwing. Also refactors File Type Detection tests to use it.for
instead of conditionals to eliminate anti-pattern.

* [feat] use destructuring with default value for readonly prop - addresses @DrJKL's Vue best practice suggestion

Replace manual fallback expressions like `readonly || false` with modern Vue 3
destructuring pattern: `const { readonly = false } = defineProps()`.
This is cleaner than withDefaults() and follows current Vue best practices.

* [test] improve test utilities usage - addresses @DrJKL's additional suggestions

- Replace findComponent with getComponent for better error handling
- Use optional chaining (?.()) instead of conditional checks for cleaner syntax
- Remove unnecessary existence checks since getComponent throws on failure

src/renderer/extensions/vueNodes/widgets/components/WidgetFileUpload.vue

@@ -187,7 +187,11 @@ import { useWidgetValue } from '@/composables/graph/useWidgetValue'
 import { useTransformCompatOverlayProps } from '@/composables/useTransformCompatOverlayProps'
 import type { SimplifiedWidget } from '@/types/simplifiedWidget'
 
-const props = defineProps<{
+const {
+  widget,
+  modelValue,
+  readonly = false
+} = defineProps<{
   widget: SimplifiedWidget<File[] | null>
   modelValue: File[] | null
   readonly?: boolean
@@ -198,8 +202,8 @@ const emit = defineEmits<{
 }>()
 
 const { localValue, onChange } = useWidgetValue({
-  widget: props.widget,
-  modelValue: props.modelValue,
+  widget,
+  modelValue,
   defaultValue: null,
   emit
 })
@@ -280,7 +284,7 @@ const triggerFileInput = () => {
 
 const handleFileChange = (event: Event) => {
   const target = event.target as HTMLInputElement
-  if (!props.readonly && target.files && target.files.length > 0) {
+  if (!readonly && target.files && target.files.length > 0) {
     // Since we only support single file, take the first one
     const file = target.files[0]