api: Add localMessageId to MessageEvent

This will be used for local echoing.  local_message_id on message
event is not yet fully documented; see CZO discussion:
  https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/local_id.2C.20queue_id.2Fsender_queue_id/near/2135340

Author: PIG208

lib/api/model/events.dart

@@ -682,7 +682,14 @@ class MessageEvent extends Event {
   @JsonKey(readValue: _readMessageValue, includeToJson: false)
   final Message message;
 
-  MessageEvent({required super.id, required this.message});
+  // When present, this equals the "local_id" parameter
+  // from a previous [sendMessage] call by us.
+  //
+  // This is not yet fully documented.  See CZO discussion for reference:
+  //   https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/local_id.2C.20queue_id.2Fsender_queue_id/near/2135340
+  final String? localMessageId;
+
+  MessageEvent({required super.id, required this.message, required this.localMessageId});
 
   static Map<String, dynamic> _readMessageValue(Map<dynamic, dynamic> json, String key) =>
     {...json['message'] as Map<String, dynamic>, 'flags': json['flags']};

lib/api/model/events.g.dart

@@ -39,6 +39,7 @@ extension SubscriptionUpdateEventChecks on Subject<SubscriptionUpdateEvent> {
 
 extension MessageEventChecks on Subject<MessageEvent> {
   Subject<Message> get message => has((e) => e.message, 'message');
+  Subject<String?> get localMessageId => has((e) => e.localMessageId, 'localMessageId');
 }
 
 extension UpdateMessageEventChecks on Subject<UpdateMessageEvent> {

test/api/model/events_checks.dart

@@ -623,7 +623,7 @@ UserTopicEvent userTopicEvent(
 }
 
 MessageEvent messageEvent(Message message) =>
-  MessageEvent(id: 0, message: message);
+  MessageEvent(id: 0, message: message, localMessageId: null);
 
 DeleteMessageEvent deleteMessageEvent(List<StreamMessage> messages) {
   assert(messages.isNotEmpty);