Fix inconsistent subgraphNode usage

Prior to this commit, subgraphNode inconsistently refers to either the
parent graph, or to indicate the current node is a subgraph.

This corrects the usage of subgraphNode to consistently refer to the
subgraph instance as defined in the constructor.

This solves a bug where graph serialization fails due to an incorrectly
reported infinite loop.

Port of https://github.com/Comfy-Org/litegraph.js/pull/1193

src/lib/litegraph/src/subgraph/ExecutableNodeDTO.ts

@@ -126,11 +126,8 @@ export class ExecutableNodeDTO implements ExecutableLGraphNode {
 
   /** Returns either the DTO itself, or the DTOs of the inner nodes of the subgraph. */
   getInnerNodes(): ExecutableLGraphNode[] {
-    return this.subgraphNode
-      ? this.subgraphNode.getInnerNodes(
-          this.nodesByExecutionId,
-          this.subgraphNodePath
-        )
+    return this.node.isSubgraphNode()
+      ? this.node.getInnerNodes(this.nodesByExecutionId, this.subgraphNodePath)
       : [this]
   }