Add JSON loading and serialization methods to PromptBuilder

lib/src/prompt_builder.dart

@@ -391,4 +391,37 @@ class PromptBuilder {
         return [];
     }
   }
+
+  /// Loads nodes from a JSON map
+  void loadFromJson(Map<String, dynamic> json) {
+    _nodes.clear();
+    _outputToNode.clear();
+
+    json.forEach((nodeId, nodeData) {
+      _nodes[nodeId] = nodeData;
+
+      // Register outputs of this node
+      final classType = nodeData["class_type"];
+      final defaultOutputs = _getDefaultOutputs(classType);
+      for (var i = 0; i < defaultOutputs.length; i++) {
+        final outputTag = nodeData["_meta"]?["outputTags"]
+                ?[defaultOutputs[i]] ??
+            defaultOutputs[i];
+        _outputToNode[outputTag] = {
+          "nodeId": nodeId,
+          "outputIndex": i.toString()
+        };
+      }
+    });
+
+    // Update the node ID counter to avoid collisions
+    _nodeIdCounter =
+        _nodes.keys.map(int.parse).fold(0, (max, id) => id > max ? id : max) +
+            1;
+  }
+
+  /// Converts the current nodes to a JSON map
+  Map<String, dynamic> toJson() {
+    return Map<String, dynamic>.from(_nodes);
+  }
 }