Fix empty combo widget for missing models in vue (#6555)

When a loaded workflow uses a model which does not exist, the combo
widget would previously display as empty. This PR
- Sets the placeholder value to the localValue, so that an invalid
selection still displays
- Sets the selection as invalid when the value is not in valid options
so that it displays in red.

<img width="817" height="200" alt="image"
src="https://github.com/user-attachments/assets/be7d6372-3957-4fd6-ac1e-778f058ec0ba"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6555-Austin-vue-combo-missing-2a06d73d36508181a96ddd5c604efba4)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

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

@@ -2,6 +2,7 @@
   <WidgetLayoutField :widget>
     <Select
       v-model="localValue"
+      :invalid
       :options="selectOptions"
       v-bind="combinedProps"
       :class="cn(WidgetInputBaseClass, 'w-full text-xs')"
@@ -53,11 +54,6 @@ const { localValue, onChange } = useWidgetValue({
 // Transform compatibility props for overlay positioning
 const transformCompatProps = useTransformCompatOverlayProps()
 
-const combinedProps = computed(() => ({
-  ...filterWidgetProps(props.widget.options, PANEL_EXCLUDED_PROPS),
-  ...transformCompatProps.value
-}))
-
 // Extract select options from widget options
 const selectOptions = computed(() => {
   const options = props.widget.options
@@ -68,4 +64,13 @@ const selectOptions = computed(() => {
 
   return []
 })
+const invalid = computed(
+  () => !!localValue.value && !selectOptions.value.includes(localValue.value)
+)
+
+const combinedProps = computed(() => ({
+  ...filterWidgetProps(props.widget.options, PANEL_EXCLUDED_PROPS),
+  ...transformCompatProps.value,
+  ...(invalid.value ? { placeholder: `${localValue.value}` } : {})
+}))
 </script>