Fix CreateMessageRule.Role to be of type Role (#264)

src/ModelContextProtocol/Client/McpClientExtensions.cs

@@ -926,7 +926,7 @@ public static class McpClientExtensions
         {
             Content = content,
             Model = chatResponse.ModelId ?? "unknown",
-            Role = lastMessage?.Role == ChatRole.User ? "user" : "assistant",
+            Role = lastMessage?.Role == ChatRole.User ? Role.User : Role.Assistant,
             StopReason = chatResponse.FinishReason == ChatFinishReason.Length ? "maxTokens" : "endTurn",
         };
     }

src/ModelContextProtocol/Protocol/Types/CreateMessageResult.cs

@@ -50,5 +50,5 @@ public class CreateMessageResult
     /// Gets or sets the role of the user who generated the message.
     /// </summary>
     [JsonPropertyName("role")]
-    public required string Role { get; init; }
+    public required Role Role { get; init; }
 }

src/ModelContextProtocol/Server/McpServerExtensions.cs

@@ -142,7 +142,7 @@ public static class McpServerExtensions
                 ModelPreferences = modelPreferences,
             }, cancellationToken).ConfigureAwait(false);
 
-        return new(new ChatMessage(new(result.Role), [result.Content.ToAIContent()]))
+        return new(new ChatMessage(result.Role is Role.User ? ChatRole.User : ChatRole.Assistant, [result.Content.ToAIContent()]))
         {
             ModelId = result.Model,
             FinishReason = result.StopReason switch