api [nfc]: Rationalize "orig"/"new" field names on UpdateMessageEvent

In the server API here, "stream_id" is the old value and contrasts
with "new_stream_id", but "subject" is the new value and contrasts
with "orig_subject".

This makes them confusing to read, and is the way it is only for
historical reasons: I believe the story is that before Zulip
Server 3.0 added support for moving topics between streams, three
of those properties were already present and then "new_stream_id"
was squeezed in at the best name still available.

Fixing that in the API will be a pain because it'll need a migration
path for compatibility.  But we can fix it here within our code.

Author: gnprice

lib/api/model/events.dart

@@ -617,24 +617,31 @@ class UpdateMessageEvent extends Event {
   final bool? renderingOnly; // TODO(server-5)
   final int messageId;
   final List<int> messageIds;
+
   final List<MessageFlag> flags;
   final int? editTimestamp; // TODO(server-5)
+
   // final String? streamName; // ignore
-  final int? streamId;
+
+  @JsonKey(name: 'stream_id')
+  final int? origStreamId;
   final int? newStreamId;
+
   final PropagateMode? propagateMode;
 
   @JsonKey(name: 'orig_subject')
   final String? origTopic;
   @JsonKey(name: 'subject')
-  final String? topic;
+  final String? newTopic;
 
   // final List<TopicLink> topicLinks; // TODO handle
+
   final String? origContent;
   final String? origRenderedContent;
   // final int? prevRenderedContentVersion; // deprecated
   final String? content;
   final String? renderedContent;
+
   final bool? isMeMessage;
 
   UpdateMessageEvent({
@@ -645,11 +652,11 @@ class UpdateMessageEvent extends Event {
     required this.messageIds,
     required this.flags,
     required this.editTimestamp,
-    required this.streamId,
+    required this.origStreamId,
     required this.newStreamId,
     required this.propagateMode,
     required this.origTopic,
-    required this.topic,
+    required this.newTopic,
     required this.origContent,
     required this.origRenderedContent,
     required this.content,

lib/api/model/events.g.dart

@@ -34,11 +34,11 @@ extension UpdateMessageEventChecks on Subject<UpdateMessageEvent> {
   Subject<List<int>> get messageIds => has((e) => e.messageIds, 'messageIds');
   Subject<List<MessageFlag>> get flags => has((e) => e.flags, 'flags');
   Subject<int?> get editTimestamp => has((e) => e.editTimestamp, 'editTimestamp');
-  Subject<int?> get streamId => has((e) => e.streamId, 'streamId');
+  Subject<int?> get origStreamId => has((e) => e.origStreamId, 'origStreamId');
   Subject<int?> get newStreamId => has((e) => e.newStreamId, 'newStreamId');
   Subject<PropagateMode?> get propagateMode => has((e) => e.propagateMode, 'propagateMode');
   Subject<String?> get origTopic => has((e) => e.origTopic, 'origTopic');
-  Subject<String?> get topic => has((e) => e.topic, 'topic');
+  Subject<String?> get newTopic => has((e) => e.newTopic, 'newTopic');
   Subject<String?> get origContent => has((e) => e.origContent, 'origContent');
   Subject<String?> get origRenderedContent => has((e) => e.origRenderedContent, 'origRenderedContent');
   Subject<String?> get content => has((e) => e.content, 'content');

test/api/model/events_checks.dart

@@ -80,13 +80,22 @@ void main() {
       'stream_id': eg.stream().streamId,
     };
 
-    test('orig_subject -> origTopic, subject -> topic', () {
+    test('stream_id -> origStreamId', () {
+      check(Event.fromJson({ ...baseJson,
+        'stream_id': 1,
+        'new_stream_id': 2,
+      }) as UpdateMessageEvent)
+        ..origStreamId.equals(1)
+        ..newStreamId.equals(2);
+    });
+
+    test('orig_subject -> origTopic, subject -> newTopic', () {
       check(Event.fromJson({ ...baseJson,
         'orig_subject': 'foo',
         'subject': 'bar',
       }) as UpdateMessageEvent)
         ..origTopic.equals('foo')
-        ..topic.equals('bar');
+        ..newTopic.equals('bar');
     });
   });
 

test/api/model/events_test.dart

@@ -407,11 +407,11 @@ UpdateMessageEvent updateMessageEditEvent(
     messageIds: [messageId],
     flags: flags ?? origMessage.flags,
     editTimestamp: editTimestamp ?? 1234567890, // TODO generate timestamp
-    streamId: origMessage is StreamMessage ? origMessage.streamId : null,
+    origStreamId: origMessage is StreamMessage ? origMessage.streamId : null,
     newStreamId: null,
     propagateMode: null,
     origTopic: null,
-    topic: null,
+    newTopic: null,
     origContent: 'some probably-mismatched old Markdown',
     origRenderedContent: origMessage.content,
     content: 'some probably-mismatched new Markdown',