api: Add missing includeToJson flags on some "type"/etc getters

These `JsonKey(includeToJson: true)` annotations are needed so that
the properties appear in the output of the generated toJson
implementations, like they do in the real JSON from the server.
We had the annotations on many of this kind of getter, but not all.

These types appear in responses, not requests, so the actual app
never calls these toJson methods.  But having accurate toJson methods
is handy in tests.

Author: gnprice

lib/api/model/events.dart

@@ -215,6 +215,7 @@ sealed class RealmUserEvent extends Event {
 @JsonSerializable(fieldRename: FieldRename.snake)
 class RealmUserAddEvent extends RealmUserEvent {
   @override
+  @JsonKey(includeToJson: true)
   String get op => 'add';
 
   final User person;
@@ -231,6 +232,7 @@ class RealmUserAddEvent extends RealmUserEvent {
 /// A [RealmUserEvent] with op `remove`: https://zulip.com/api/get-events#realm_user-remove
 class RealmUserRemoveEvent extends RealmUserEvent {
   @override
+  @JsonKey(includeToJson: true)
   String get op => 'remove';
 
   final int userId;
@@ -268,6 +270,7 @@ class RealmUserUpdateCustomProfileField {
 @JsonSerializable(fieldRename: FieldRename.snake)
 class RealmUserUpdateEvent extends RealmUserEvent {
   @override
+  @JsonKey(includeToJson: true)
   String get op => 'update';
 
   @JsonKey(readValue: _readFromPerson) final int userId;
@@ -351,6 +354,7 @@ sealed class ChannelEvent extends Event {
 @JsonSerializable(fieldRename: FieldRename.snake)
 class ChannelCreateEvent extends ChannelEvent {
   @override
+  @JsonKey(includeToJson: true)
   String get op => 'create';
 
   final List<ZulipStream> streams;
@@ -368,6 +372,7 @@ class ChannelCreateEvent extends ChannelEvent {
 @JsonSerializable(fieldRename: FieldRename.snake)
 class ChannelDeleteEvent extends ChannelEvent {
   @override
+  @JsonKey(includeToJson: true)
   String get op => 'delete';
 
   final List<ZulipStream> streams;
@@ -385,6 +390,7 @@ class ChannelDeleteEvent extends ChannelEvent {
 @JsonSerializable(fieldRename: FieldRename.snake)
 class ChannelUpdateEvent extends ChannelEvent {
   @override
+  @JsonKey(includeToJson: true)
   String get op => 'update';
 
   final int streamId;
@@ -666,6 +672,7 @@ class UserTopicEvent extends Event {
 //   in order to skip the boilerplate in [fromJson] and [toJson].
 class MessageEvent extends Event {
   @override
+  @JsonKey(includeToJson: true)
   String get type => 'message';
 
   // In the server API, the `flags` field appears directly on the event rather
@@ -857,6 +864,7 @@ sealed class UpdateMessageFlagsEvent extends Event {
 @JsonSerializable(fieldRename: FieldRename.snake)
 class UpdateMessageFlagsAddEvent extends UpdateMessageFlagsEvent {
   @override
+  @JsonKey(includeToJson: true)
   String get op => 'add';
 
   final bool all;
@@ -879,6 +887,7 @@ class UpdateMessageFlagsAddEvent extends UpdateMessageFlagsEvent {
 @JsonSerializable(fieldRename: FieldRename.snake)
 class UpdateMessageFlagsRemoveEvent extends UpdateMessageFlagsEvent {
   @override
+  @JsonKey(includeToJson: true)
   String get op => 'remove';
 
   // final bool all; // deprecated, ignore