Serialize structured content not result

Author: PederHP

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -263,7 +263,7 @@ public override async ValueTask<CallToolResult> InvokeAsync(
                 // If there is structuredOutput we must stringify it, as the MCP specification requires Content to be for backwards compatibility
                 // but otherwise not differ
                 Content = [new TextContentBlock { Text = structuredContent == null ? text :
-                 JsonSerializer.Serialize(result, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object)))}],
+                 JsonSerializer.Serialize(structuredContent, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object)))}],
                 StructuredContent = structuredContent,
             },