Subgraph widget promotion - Part 2 (#5617)

Implements proxyWidget support on subgraph nodes.  This registers a
special proxyWidgets property on subgraph nodes which is directly mapped
to the proxyWidgets displayed on the node. Each proxyWidget directly
maps to a real widget inside the subgraph.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5617-Subgraph-widget-promotion-Part-2-2716d73d3650813d8621fefdce6ae518)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

src/core/schemas/proxyWidget.ts

@@ -0,0 +1,23 @@
+import { z } from 'zod'
+import { fromZodError } from 'zod-validation-error'
+
+import type { NodeProperty } from '@/lib/litegraph/src/LGraphNode'
+
+const proxyWidgetsPropertySchema = z.array(z.tuple([z.string(), z.string()]))
+type ProxyWidgetsProperty = z.infer<typeof proxyWidgetsPropertySchema>
+
+export function parseProxyWidgets(
+  property: NodeProperty | undefined
+): ProxyWidgetsProperty {
+  if (typeof property !== 'string') {
+    throw new Error(
+      'Invalid assignment for properties.proxyWidgets:\nValue must be a string'
+    )
+  }
+  const parsed = JSON.parse(property)
+  const result = proxyWidgetsPropertySchema.safeParse(parsed)
+  if (result.success) return result.data
+
+  const error = fromZodError(result.error)
+  throw new Error(`Invalid assignment for properties.proxyWidgets:\n${error}`)
+}