[backport 1.25] Fix subgraph reroute serialization (#4920)

Co-authored-by: AustinMroz <austin@comfy.org>

src/lib/litegraph/src/LGraph.ts

@@ -2331,6 +2331,9 @@ export class Subgraph
       nodes: this.nodes.map((node) => node.serialize()),
       groups: this.groups.map((group) => group.serialize()),
       links: [...this.links.values()].map((x) => x.asSerialisable()),
+      reroutes: this.reroutes.size
+        ? [...this.reroutes.values()].map((x) => x.asSerialisable())
+        : undefined,
       extra: this.extra
     }
   }