api: Assert that unread dm and stream snapshots have sorted message IDs

See discussion, which should produce a docs change soon:
  https://chat.zulip.org/#narrow/stream/412-api-documentation/topic/register.3A.20.60unread_message_ids.60.20sorted.3F/near/1647932

Author: chrisbobbe

lib/api/model/initial_snapshot.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:json_annotation/json_annotation.dart';
 
+import '../../model/algorithms.dart';
 import 'model.dart';
 
 part 'initial_snapshot.g.dart';
@@ -263,7 +264,7 @@ class UnreadDmSnapshot {
   UnreadDmSnapshot({
     required this.otherUserId,
     required this.unreadMessageIds,
-  });
+  }) : assert(isSortedWithoutDuplicates(unreadMessageIds));
 
   factory UnreadDmSnapshot.fromJson(Map<String, dynamic> json) =>
     _$UnreadDmSnapshotFromJson(json);
@@ -282,7 +283,7 @@ class UnreadStreamSnapshot {
     required this.topic,
     required this.streamId,
     required this.unreadMessageIds,
-  });
+  }) : assert(isSortedWithoutDuplicates(unreadMessageIds));
 
   factory UnreadStreamSnapshot.fromJson(Map<String, dynamic> json) =>
     _$UnreadStreamSnapshotFromJson(json);
@@ -299,7 +300,7 @@ class UnreadHuddleSnapshot {
   UnreadHuddleSnapshot({
     required this.userIdsString,
     required this.unreadMessageIds,
-  });
+  }) : assert(isSortedWithoutDuplicates(unreadMessageIds));
 
   factory UnreadHuddleSnapshot.fromJson(Map<String, dynamic> json) =>
     _$UnreadHuddleSnapshotFromJson(json);

test/api/model/initial_snapshot_test.dart

@@ -0,0 +1,43 @@
+import 'package:checks/checks.dart';
+import 'package:test/scaffolding.dart';
+import 'package:zulip/api/model/initial_snapshot.dart';
+
+void main() {
+  test('UnreadDmSnapshot: require sorted unreadMessageIds', () {
+    check(() => UnreadDmSnapshot.fromJson({
+      'other_user_id': 1,
+      'unread_message_ids': [1, 2, 3],
+    })).returnsNormally();
+
+    check(() => UnreadDmSnapshot.fromJson({
+      'other_user_id': 1,
+      'unread_message_ids': [11, 2, 3],
+    })).throws<AssertionError>();
+  });
+
+  test('UnreadStreamSnapshot: require sorted unreadMessageIds', () {
+    check(() => UnreadStreamSnapshot.fromJson({
+      'topic': 'a',
+      'stream_id': 1,
+      'unread_message_ids': [1, 2, 3],
+    })).returnsNormally();
+
+    check(() => UnreadStreamSnapshot.fromJson({
+      'topic': 'a',
+      'stream_id': 1,
+      'unread_message_ids': [11, 2, 3],
+    })).throws<AssertionError>();
+  });
+
+  test('UnreadHuddleSnapshot: require sorted unreadMessageIds', () {
+    check(() => UnreadHuddleSnapshot.fromJson({
+      'user_ids_string': '1,2',
+      'unread_message_ids': [1, 2, 3],
+    })).returnsNormally();
+
+    check(() => UnreadHuddleSnapshot.fromJson({
+      'user_ids_string': '1,2',
+      'unread_message_ids': [11, 2, 3],
+    })).throws<AssertionError>();
+  });
+}