api: Explicitly ignore topics on DMs

The topic/subject field on DMs carries no information in the current
API, and we already never actually look at it, as demonstrated by
the lack of changes in the rest of the tree in this commit.  Clean
up the type definitions in the API a bit by making that explicit.

This is NFC except in the case where a server were to not send this
field at all for a DM (or to send a non-string value).  In that case
the old code would reject the server's response as malformed, and
the new code after this commit would accept it because it doesn't
look for the field.

Author: gnprice

lib/api/model/model.dart

@@ -556,11 +556,11 @@ sealed class Message {
   final String senderFullName;
   final int senderId;
   final String senderRealmStr;
-  @JsonKey(name: 'subject')
-  String topic;
+
   /// Poll data if "submessages" describe a poll, `null` otherwise.
   @JsonKey(name: 'submessages', readValue: _readPoll, fromJson: Poll.fromJson, toJson: Poll.toJson)
   Poll? poll;
+
   final int timestamp;
   String get type;
 
@@ -613,7 +613,6 @@ sealed class Message {
     required this.senderFullName,
     required this.senderId,
     required this.senderRealmStr,
-    required this.topic,
     required this.timestamp,
     required this.flags,
     required this.matchContent,
@@ -667,8 +666,16 @@ class StreamMessage extends Message {
   // invalidated.
   @JsonKey(required: true, disallowNullValue: true)
   String? displayRecipient;
+
   int streamId;
 
+  // The topic/subject is documented to be present on DMs too, just empty.
+  // We ignore it on DMs; if a future server introduces distinct topics in DMs,
+  // that will need new UI that we'll design then as part of that feature,
+  // and ignoring the topics seems as good a fallback behavior as any.
+  @JsonKey(name: 'subject')
+  String topic;
+
   StreamMessage({
     required super.client,
     required super.content,
@@ -683,13 +690,13 @@ class StreamMessage extends Message {
     required super.senderFullName,
     required super.senderId,
     required super.senderRealmStr,
-    required super.topic,
     required super.timestamp,
     required super.flags,
     required super.matchContent,
     required super.matchTopic,
     required this.displayRecipient,
     required this.streamId,
+    required this.topic,
   });
 
   factory StreamMessage.fromJson(Map<String, dynamic> json) =>
@@ -786,7 +793,6 @@ class DmMessage extends Message {
     required super.senderFullName,
     required super.senderId,
     required super.senderRealmStr,
-    required super.topic,
     required super.timestamp,
     required super.flags,
     required super.matchContent,

lib/api/model/model.g.dart

@@ -633,6 +633,7 @@ class QuoteAndReplyButton extends MessageActionSheetMenuItemButton {
 
   @override void onPressed() async {
     final zulipLocalizations = ZulipLocalizations.of(pageContext);
+    final message = this.message;
 
     var composeBoxController = findMessageListPage().composeBoxController;
     // The compose box doesn't null out its controller; it's either always null

lib/widgets/action_sheet.dart

@@ -38,7 +38,6 @@ extension MessageChecks on Subject<Message> {
   Subject<String> get senderFullName => has((e) => e.senderFullName, 'senderFullName');
   Subject<int> get senderId => has((e) => e.senderId, 'senderId');
   Subject<String> get senderRealmStr => has((e) => e.senderRealmStr, 'senderRealmStr');
-  Subject<String> get topic => has((e) => e.topic, 'topic');
   Subject<Poll?> get poll => has((e) => e.poll, 'poll');
   Subject<int> get timestamp => has((e) => e.timestamp, 'timestamp');
   Subject<String> get type => has((e) => e.type, 'type');
@@ -49,6 +48,7 @@ extension MessageChecks on Subject<Message> {
 
 extension StreamMessageChecks on Subject<StreamMessage> {
   Subject<String?> get displayRecipient => has((e) => e.displayRecipient, 'displayRecipient');
+  Subject<String> get topic => has((e) => e.topic, 'topic');
 }
 
 extension ReactionsChecks on Subject<Reactions> {

test/api/model/model_checks.dart

@@ -89,7 +89,8 @@ void main() {
       check(baseStreamJson()).not((it) => it.containsKey('topic'));
       check(Message.fromJson(baseStreamJson()
         ..['subject'] = 'hello'
-      )).topic.equals('hello');
+      )).isA<StreamMessage>()
+        .topic.equals('hello');
     });
 
     test('match_subject -> matchTopic', () {