Fix edge cases in subgraph removal logic (#8758)

#8187 made removal of subgraphs cleanup the subgraph definition for the
removed graph and call onRemove handlers. However, it missed some edge
cases and broke subgraph conversion of selections containing subgraphs
which this PR tries to address.
- Deeply nested subgraphs are now also cleaned
- Adding a subgraphNode to the graph also ensures nested subgraphs are
added to subgraph definitions

Reminder: under this change, nodes can continue to exist after their
onRemoved handler has been called
- It may be better to instead perform the "garbage collection" of
subgraphs outside of the graph removal step to better handle edge cases
like subgraph conversion where a subgraph may continue to persist after
a parent subgraphNode has been removed from a graph.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8758-Fix-edge-cases-in-subgraph-removal-logic-3026d73d36508177b34ffdd2e0a114fe)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/LGraph.ts

@@ -8,6 +8,7 @@ import type { UUID } from '@/lib/litegraph/src/utils/uuid'
 import { createUuidv4, zeroUuid } from '@/lib/litegraph/src/utils/uuid'
 import { useLayoutMutations } from '@/renderer/core/layout/operations/layoutMutations'
 import { LayoutSource } from '@/renderer/core/layout/types'
+import { forEachNode } from '@/utils/graphTraversalUtil'
 
 import type { DragAndScaleState } from './DragAndScale'
 import { LGraphCanvas } from './LGraphCanvas'
@@ -973,6 +974,13 @@ export class LGraph
       this.canvasAction((c) => c.startGhostPlacement(node, opts.dragEvent))
     }
 
+    if (node.isSubgraphNode?.()) {
+      forEachNode(node.subgraph, (innerNode) => {
+        if (innerNode.isSubgraphNode())
+          this.subgraphs.set(innerNode.subgraph.id, innerNode.subgraph)
+      })
+    }
+
     // to chain actions
     return node
   }
@@ -1034,14 +1042,14 @@ export class LGraph
       }
     }
 
-    // Subgraph cleanup (use local const to avoid type narrowing affecting node.graph assignment)
-    const subgraphNode = node.isSubgraphNode() ? node : null
-    if (subgraphNode) {
-      for (const innerNode of subgraphNode.subgraph.nodes) {
+    if (node.isSubgraphNode()) {
+      forEachNode(node.subgraph, (innerNode) => {
         innerNode.onRemoved?.()
-        subgraphNode.subgraph.onNodeRemoved?.(innerNode)
-      }
-      this.rootGraph.subgraphs.delete(subgraphNode.subgraph.id)
+        innerNode.graph?.onNodeRemoved?.(innerNode)
+        if (innerNode.isSubgraphNode())
+          this.rootGraph.subgraphs.delete(innerNode.subgraph.id)
+      })
+      this.rootGraph.subgraphs.delete(node.subgraph.id)
     }
 
     // callback