submodule对gitea不管用，所以直接拉了一份拉格兰

Lagrange.Core/Lagrange.OneBot/Message/Entity/MarkdownSegment.cs

@@ -0,0 +1,27 @@
+using System.Text.Json.Serialization;
+using Lagrange.Core.Message;
+using Lagrange.Core.Message.Entity;
+
+namespace Lagrange.OneBot.Message.Entity;
+
+[Serializable]
+public partial class MarkdownSegment(string content)
+{
+    public MarkdownSegment(): this("") { }
+    
+    [JsonPropertyName("content")] [CQProperty] public string Content { get; set; } = content;
+}
+
+[SegmentSubscriber(typeof(MarkdownEntity), "markdown")]
+public partial class MarkdownSegment : SegmentBase
+{
+    public override void Build(MessageBuilder builder, SegmentBase segment)
+    {
+        if (segment is MarkdownSegment markdown) builder.Markdown(markdown.Content);
+    }
+
+    public override SegmentBase? FromEntity(MessageChain chain, IMessageEntity entity)
+    {
+        return null;
+    }
+}