fix: skip PromotedWidgetSlot when setting promoted flag in subgraph-opened handler

PromotedWidgetSlot overrides promoted as a getter-only property, causing a TypeError when the subgraph-opened handler tried to set it.

Amp-Thread-ID: https://ampcode.com/threads/T-019c5585-480f-7523-96a7-f624a6689c1e
Co-authored-by: Amp <amp@ampcode.com>

src/core/graph/subgraph/promotedWidgetRegistration.ts

@@ -29,6 +29,7 @@ export function registerPromotedWidgetSlots(canvas: LGraphCanvas) {
     const proxyWidgets = parseProxyWidgets(fromNode.properties.proxyWidgets)
     for (const node of subgraph.nodes) {
       for (const widget of node.widgets ?? []) {
+        if (widget instanceof PromotedWidgetSlot) continue
         widget.promoted = proxyWidgets.some(
           ([n, w]) => node.id == n && widget.name == w
         )