api: Add send-submessage route

The route is not documented in the Zulip API Documentation.

Instead, we refer to the zulip-mobile implementation and
https://zulip.readthedocs.io/en/latest/subsystems/widgets.html#polls-todo-lists-and-games.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/api/model/events.g.dart

@@ -82,9 +82,12 @@ class Submessage {
 /// The only type of submessage that actually exists in Zulip (as of 2024,
 /// and since this "submessages" subsystem was created in 2017–2018)
 /// is [SubmessageType.widget].
+@JsonEnum(fieldRename: FieldRename.snake, alwaysCreate: true)
 enum SubmessageType {
   widget,
-  unknown,
+  unknown;
+
+  String toJson() => _$SubmessageTypeEnumMap[this]!;
 }
 
 /// The data encoded in a submessage at [Submessage.content].

lib/api/model/submessage.dart

@@ -0,0 +1,16 @@
+import '../core.dart';
+import '../model/submessage.dart';
+
+/// https://zulip.readthedocs.io/en/latest/subsystems/widgets.html#polls-todo-lists-and-games
+/// https://github.com/zulip/zulip-mobile/blob/fc23edd67a5ec7f32c7c5f6cd81893b94dc043a2/src/api/submessages/sendSubmessage.js
+Future<void> sendSubmessage(ApiConnection connection, {
+  required int messageId,
+  required SubmessageType submessageType,
+  required SubmessageData content,
+}) {
+  return connection.post('sendSubmessage', (_) {}, 'submessage', {
+    'message_id': messageId,
+    'msg_type': RawParameter(submessageType.toJson()),
+    'content': content,
+  });
+}

lib/api/model/submessage.g.dart

@@ -0,0 +1,27 @@
+import 'package:checks/checks.dart';
+import 'package:http/http.dart' as http;
+import 'package:flutter_test/flutter_test.dart';
+import 'package:zulip/api/model/submessage.dart';
+import 'package:zulip/api/route/submessage.dart';
+
+import '../../stdlib_checks.dart';
+import '../fake_api.dart';
+
+void main() {
+  test('smoke sendSubmessage', () {
+    return FakeApiConnection.with_<void>((connection) async {
+      connection.prepare(json: {});
+      await sendSubmessage(connection, messageId: 1,
+        submessageType: SubmessageType.widget,
+        content: PollQuestionEventSubmessage(question: 'test question'));
+      check(connection.takeRequests()).single.isA<http.Request>()
+        ..method.equals('POST')
+        ..url.path.equals('/api/v1/submessage')
+        ..bodyFields.deepEquals({
+          'message_id': '1',
+          'msg_type': 'widget',
+          'content': '{"type":"question","question":"test question"}',
+        });
+    });
+  });
+}