fix: prevent non-widget inputs on nested subgraphs from appearing as button widgets (#9542)

## Summary

Fix non-widget inputs on nested subgraphs appearing twice — once as
slots and once as unresolved button widgets.

## Changes

- **What**: Add `getTargetWidget()` guard in the `isSubgraphNode()`
branch of `resolveSubgraphInputTarget`, matching the existing check for
non-subgraph nodes. Non-widget inputs (e.g. AUDIO, IMAGE) now return
`undefined` instead of a bogus promotion entry.

## Review Focus

`resolveSubgraphInputTarget` had an asymmetry: the non-subgraph branch
checked `getTargetWidget()` before returning, but the `isSubgraphNode()`
branch returned unconditionally for every input. For nested subgraphs
where non-widget slots are linked through to inner SubgraphNode inputs,
this created `PromotedWidgetView` entries that failed `resolveDeepest()`
(falling back to `type: 'button'`), while the inputs also rendered as
normal slot circles since `input.widget` was never set by
`_resolveInputWidget` (which correctly skipped them).

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9542-fix-prevent-non-widget-inputs-on-nested-subgraphs-from-appearing-as-button-widgets-31c6d73d3650816387c3f97f0385e762)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>

src/core/graph/subgraph/resolveSubgraphInputTarget.ts

@@ -16,6 +16,9 @@ export function resolveSubgraphInputTarget(
     inputName,
     ({ inputNode, targetInput, getTargetWidget }) => {
       if (inputNode.isSubgraphNode()) {
+        const targetWidget = getTargetWidget()
+        if (!targetWidget) return undefined
+
         return {
           nodeId: String(inputNode.id),
           widgetName: targetInput.name