api [nfc]: Use TopicName type across request parameters

This completes the conversion to use the TopicName type in the
places where it belongs.

Next, we'll start making use of this new type distinction: instead
of using a TopicName implicitly as a String, we'll introduce members
corresponding to the different ways a topic name is used which are
going to start having different behavior in the future.

After that's complete, we can start enforcing the distinction in
order to catch accidental future implicit uses as a String.

Author: gnprice

lib/api/route/channels.dart

@@ -46,7 +46,7 @@ class GetStreamTopicsEntry {
 // TODO(server-7): remove this and just use updateUserTopic
 Future<void> updateUserTopicCompat(ApiConnection connection, {
   required int streamId,
-  required String topic,
+  required TopicName topic,
   required UserTopicVisibilityPolicy visibilityPolicy,
 }) {
   final useLegacyApi = connection.zulipFeatureLevel! < 170;
@@ -76,7 +76,7 @@ Future<void> updateUserTopicCompat(ApiConnection connection, {
 // TODO(server-7) remove FL 170+ mention in doc, and the related `assert`
 Future<void> updateUserTopic(ApiConnection connection, {
   required int streamId,
-  required String topic,
+  required TopicName topic,
   required UserTopicVisibilityPolicy visibilityPolicy,
 }) {
   assert(visibilityPolicy != UserTopicVisibilityPolicy.unknown);

lib/api/route/messages.dart

@@ -231,7 +231,7 @@ class StreamDestination extends MessageDestination {
   const StreamDestination(this.streamId, this.topic);
 
   final int streamId;
-  final String topic;
+  final TopicName topic;
 }
 
 /// A DM conversation, for specifying to [sendMessage].
@@ -449,7 +449,7 @@ Future<void> markStreamAsRead(ApiConnection connection, {
 // TODO(server-6): Remove as deprecated by updateMessageFlagsForNarrow
 Future<void> markTopicAsRead(ApiConnection connection, {
   required int streamId,
-  required String topicName,
+  required TopicName topicName,
 }) {
   return connection.post('markTopicAsRead', (_) {}, 'mark_topic_as_read', {
     'stream_id': streamId,