Create and use an enum for AIRole in Scribe

scribe/lib/scribe/ai/data/model/ai_message.dart

@@ -2,12 +2,16 @@ import 'package:json_annotation/json_annotation.dart';
 
 part 'ai_message.g.dart';
 
+enum AIRole {
+  @JsonValue('user')
+  user,
+  @JsonValue('system')
+  system,
+}
+
 @JsonSerializable()
 class AIMessage {
-  static const String aiUserRole = 'user';
-  static const String aiSystemRole = 'system';
-
-  final String role;
+  final AIRole role;
   final String content;
 
   const AIMessage({
@@ -21,12 +25,12 @@ class AIMessage {
   Map<String, dynamic> toJson() => _$AIMessageToJson(this);
 
   factory AIMessage.ofUser(String content) => AIMessage(
-        role: aiUserRole,
+        role: AIRole.user,
         content: content,
       );
 
   factory AIMessage.ofSystem(String content) => AIMessage(
-        role: aiSystemRole,
+        role: AIRole.system,
         content: content,
       );
 }

scribe/lib/scribe/ai/domain/model/prompt_data.dart

@@ -54,9 +54,9 @@ class Prompt {
   List<AIMessage> buildPrompt(String inputText, {String? task}) {
     final messages = <AIMessage>[];
     for (final message in this.messages) {
-      if (message.role == 'system') {
+      if (message.role == AIRole.system) {
         messages.add(AIMessage.ofSystem(message.content));
-      } else if (message.role == 'user') {
+      } else if (message.role == AIRole.user) {
         final userContent = _replacePlaceholders(message.content, inputText, task);
         messages.add(AIMessage.ofUser(userContent));
       }