Add JSON serialization methods to PromptBuilder for improved data handling

lib/src/prompt_builder.dart

@@ -1,10 +1,31 @@
 class PromptBuilder {
-  final String clientId;
+  String clientId;
   final Map<String, dynamic> _nodes = {};
-  final Map<String, Map<String, String>> _outputToNode = {};
+  final Map<String, Map<String, dynamic>> _outputToNode = {};
   int _nodeIdCounter = 1;
 
   PromptBuilder({required this.clientId});
+  Map<String, dynamic> toJson() {
+    return {
+      "nodes": _nodes,
+      "outputToNode": _outputToNode,
+      "nodeIdCounter": _nodeIdCounter
+    };
+  }
+
+  void fromJson(Map<String, dynamic> json) {
+    _nodes.clear();
+    _outputToNode.clear();
+    _nodeIdCounter = json["nodeIdCounter"];
+
+    for (var entry in json["nodes"].entries) {
+      _nodes[entry.key] = entry.value;
+    }
+
+    for (var entry in json["outputToNode"].entries) {
+      _outputToNode[entry.key] = entry.value;
+    }
+  }
 
   /// Adds a node to the workflow and returns its nodeId
   String addNode(String classType, Map<String, dynamic> inputs,