model [nfc]: Implement ReactionType.toJson

We'll call this directly soon, when we'll want reusable code to
translate an enum value into a 'snake_case' string, for API bindings
for the add- and remove-reaction endpoints.

With a `toJson` method, instances can now be handled by
`encodeJson`. That's nice, but not really a goal here; we don't have
an imminent need to pass an instance through `encodeJson`. Anyway,
it means that json_serializable's generated _$ReactionToJson and
_$ReactionEventToJson functions don't need the enum-to-string
conversion on their reactionType fields in order for jsonEncode to
accept those functions' output. And it looks like json_serializable
has removed that conversion in both functions. So, adjust our code
to not depend on it.

Author: chrisbobbe

lib/api/model/events.g.dart

@@ -475,4 +475,6 @@ enum ReactionType {
   unicodeEmoji,
   realmEmoji,
   zulipExtraEmoji;
+
+  String toJson() => _$ReactionTypeEnumMap[this]!;
 }

lib/api/model/model.dart

@@ -147,7 +147,9 @@ StreamMessage streamMessage({
     ..._messagePropertiesFromContent(content, contentMarkdown),
     'display_recipient': effectiveStream.name,
     'stream_id': effectiveStream.streamId,
-    'reactions': reactions?.map((r) => r.toJson()).toList() ?? [],
+    'reactions': reactions?.map(
+      (r) => r.toJson()..['reaction_type'] = r.reactionType.toJson(),
+    ).toList() ?? [],
     'flags': flags ?? [],
     'id': id ?? 1234567, // TODO generate example IDs
     'last_edit_timestamp': lastEditTimestamp,

lib/api/model/model.g.dart

@@ -396,23 +396,22 @@ void main() async {
 
       test('remove reaction', () async {
         final eventReaction = Reaction(reactionType: ReactionType.unicodeEmoji,
-          emojiName: 'wave',                  emojiCode: '1f44b', userId: 1);
+          emojiName: 'wave',          emojiCode: '1f44b', userId: 1);
 
         // Same emoji, different user. Not to be removed.
-        final reaction2 = Reaction.fromJson(eventReaction.toJson()
-          ..['user_id'] = 2);
+        final reaction2 = Reaction(reactionType: ReactionType.unicodeEmoji,
+          emojiName: 'wave',          emojiCode: '1f44b', userId: 2);
 
         // Same user, different emoji. Not to be removed.
-        final reaction3 = Reaction.fromJson(eventReaction.toJson()
-          ..['emoji_code'] = '1f6e0'
-          ..['emoji_name'] = 'working_on_it');
+        final reaction3 = Reaction(reactionType: ReactionType.unicodeEmoji,
+          emojiName: 'working_on_it', emojiCode: '1f6e0', userId: 1);
 
         // Same user, same emojiCode, different emojiName. To be removed: servers
         // key on user, message, reaction type, and emoji code, but not emoji name.
         // So we mimic that behavior; see discussion:
         //   https://github.com/zulip/zulip-flutter/pull/256#discussion_r1284865099
-        final reaction4 = Reaction.fromJson(eventReaction.toJson()
-          ..['emoji_name'] = 'hello');
+        final reaction4 = Reaction(reactionType: ReactionType.unicodeEmoji,
+          emojiName: 'hello',         emojiCode: '1f44b', userId: 1);
 
         final originalMessage = eg.streamMessage(
           reactions: [reaction2, reaction3, reaction4]);