fix: no attachment broadcast fix

internal/service/chat_service.go

@@ -1118,6 +1118,14 @@ func (s *ChatService) systemMessage(ctx context.Context, conversationID, body st
 		return
 	}
 	if created {
+		// Publish the hydrated message. Append leaves Attachments nil, which
+		// serialises to null and breaks clients that trust the "always an
+		// array" contract this field's own doc comment promises.
+		if full, hErr := s.withAttachments(ctx, "", msg); hErr == nil {
+			msg = full
+		} else if msg.Attachments == nil {
+			msg.Attachments = []model.Attachment{}
+		}
 		s.publish(ctx, conversationID, "", msg)
 	}
 }