[backend/federation] Fix custom emoji reaction federation with Akkoma (ISH-153)

Iceshrimp.Backend/Core/Federation/ActivityStreams/Contexts/as.json

@@ -200,7 +200,8 @@
     },
     "tag": {
       "@id": "as:tag",
-      "@type": "@id"
+      "@type": "@id",
+      "@container": "@list"
     },
     "target": {
       "@id": "as:target",

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASTag.cs

@@ -46,7 +46,7 @@ public class ASEmoji : ASTag
 	[J($"{Constants.ActivityStreamsNs}#icon")]
 	[JC(typeof(ASImageConverter))]
 	public ASImage? Image { get; set; }
-	
+
 	[J($"{Constants.ActivityStreamsNs}#name")]
 	[JC(typeof(VC))]
 	public string? Name { get; set; }
@@ -54,7 +54,7 @@ public class ASEmoji : ASTag
 
 public sealed class ASTagConverter : JsonConverter
 {
-	public override bool CanWrite => false;
+	public override bool CanWrite => true;
 
 	public override bool CanConvert(Type objectType)
 	{
@@ -74,7 +74,27 @@ public sealed class ASTagConverter : JsonConverter
 
 		if (reader.TokenType == JsonToken.StartArray)
 		{
-			var array  = JArray.Load(reader);
+			// We have to use @list here because otherwise Akkoma will refuse to process activities with only one Tag.
+			// To solve this, we've patched the AS2 context to render & parse the tag type as @list, forcing it to be an array.
+			// Ref: https://akkoma.dev/AkkomaGang/akkoma/issues/720
+			var array = JArray.Load(reader);
+			if (array.Count > 0)
+			{
+				try
+				{
+					return array.SelectToken("$.[*].@list")
+					            ?.Children()
+					            .OfType<JObject>()
+					            .Select(HandleObject)
+					            .OfType<ASTag>()
+					            .ToList();
+				}
+				catch
+				{
+					//ignored
+				}
+			}
+
 			var result = new List<ASTag>();
 			foreach (var token in array)
 			{
@@ -105,6 +125,15 @@ public sealed class ASTagConverter : JsonConverter
 
 	public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
 	{
-		throw new NotImplementedException();
+		if (value == null)
+		{
+			writer.WriteNull();
+			return;
+		}
+
+		writer.WriteStartObject();
+		writer.WritePropertyName("@list");
+		serializer.Serialize(writer, value);
+		writer.WriteEndObject();
 	}
 }