submessage: Extract OptionKey.

Semantically, we expect strings of a specific shape to be the key for
options. This make it clearer to the reader without complicating things
with extra validation.

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

Author: PIG208

lib/api/model/submessage.dart

@@ -173,7 +173,7 @@ sealed class PollEventSubmessage extends SubmessageData {
   ///
   /// For options that are a part of the initial [PollWidgetData], the
   /// [senderId] should be `null`.
-  static String optionKey({required int? senderId, required int idx}) =>
+  static OptionKey optionKey({required int? senderId, required int idx}) =>
     // "canned" is a canonical constant coined by the web client:
     //   https://github.com/zulip/zulip/blob/40f59a05c/web/shared/src/poll_data.ts#L238
     '${senderId ?? 'canned'},$idx';
@@ -205,6 +205,8 @@ enum PollEventSubmessageType {
     .map((key, value) => MapEntry(value, key));
 }
 
+typedef OptionKey = String;
+
 /// A poll event when an option is added.
 ///
 /// See: https://github.com/zulip/zulip/blob/40f59a05c/web/shared/src/poll_data.ts#L112-L159
@@ -264,7 +266,7 @@ class PollVoteEventSubmessage extends PollEventSubmessage {
   /// The key of the affected option.
   ///
   /// See [PollEventSubmessage.optionKey].
-  final String key;
+  final OptionKey key;
   @JsonKey(name: 'vote', unknownEnumValue: PollVoteOp.unknown)
   final PollVoteOp op;