Fix disconnection of subgraphInput links (#6258)

`LLink.disconnect` is intended to cleanup only the link itself. #4800
mistakenly assumed that it would perform all required steps for
disconnection. Later, #5015 would partially resolve this by adding some
of the missing functionality into `LLink.disconnect`, but this still
left output cleanup unhandled and failed to call
`node.onConnectionsChanged`.

This PR instead moves the disconnection code to call the function that
already has robust handling for these items and removes the
no-longer-needed and potentially misleading workaround.

Resolves #6247

Also un-skipped several SubgraphIO tests. They appear to function fine.
I'm assuming the reasons for them being skipped have been resolved.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6258-Fix-disconnection-of-subgraphInput-links-2966d73d36508112ad1fe602cdcf461b)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/LLink.ts

@@ -418,18 +418,6 @@ export class LLink implements LinkSegment, Serialisable<SerialisableLLink> {
    * If `input` or `output`, reroutes will not be automatically removed, and retain a connection to the input or output, respectively.
    */
   disconnect(network: LinkNetwork, keepReroutes?: 'input' | 'output'): void {
-    // Clean up the target node's input slot
-    if (this.target_id !== -1) {
-      const targetNode = network.getNodeById(this.target_id)
-      if (targetNode) {
-        const targetInput = targetNode.inputs?.[this.target_slot]
-        if (targetInput && targetInput.link === this.id) {
-          targetInput.link = null
-          targetNode.setDirtyCanvas?.(true, false)
-        }
-      }
-    }
-
     const reroutes = LLink.getReroutes(network, this)
 
     const lastReroute = reroutes.at(-1)

src/lib/litegraph/src/canvas/ToInputFromIoNodeLink.ts

@@ -137,7 +137,9 @@ export class ToInputFromIoNodeLink implements RenderLink {
   }
   disconnect(): boolean {
     if (!this.existingLink) return false
-    this.existingLink.disconnect(this.network, 'input')
+    const { input, inputNode } = this.existingLink.resolve(this.network)
+    if (!inputNode || !input) return false
+    this.node._disconnectNodeInput(inputNode, input, this.existingLink)
     return true
   }
 }