Make `RichMessage` `ConstrainSingle`

mirai-core-api/src/commonMain/kotlin/message/data/RichMessage.kt

@@ -28,7 +28,8 @@ import kotlin.annotation.AnnotationTarget.*
  */
 // not using sealed class for customized implementations
 // using polymorphic serializer from Message.Serializer
-public interface RichMessage : MessageContent {
+public interface RichMessage : MessageContent, ConstrainSingle {
+    override val key: MessageKey<*> get() = Key
 
     /**
      * **注意**: 富文本消息的 [RichMessage.contentEquals] 和 [RichMessage.toString] 都不稳定. 将来可能在没有任何警告的情况下改变格式.