DEV: Refactor DM channel creation into new service pattern (#22144)

This will be used when we move the channel creation for DMs
to happen when we first send a message in a DM channel to avoid
a double-request. For now we can just have a new API endpoint
for creating this that the existing frontend code can use,
that uses the new service pattern.

This also uses the new policy pattern for services where the policy
can be defined in a class so a more dynamic reason for the policy
failing can be sent to the controller.

Co-authored-by: Loïc Guitaut <loic@discourse.org>

plugins/chat/spec/jobs/regular/chat/notify_mentioned_spec.rb

@@ -15,8 +15,15 @@ describe Jobs::Chat::NotifyMentioned do
     user_2.reload
 
     @chat_group = Fabricate(:group, users: [user_1, user_2])
-    @personal_chat_channel =
-      Chat::DirectMessageChannelCreator.create!(acting_user: user_1, target_users: [user_1, user_2])
+    result =
+      Chat::CreateDirectMessageChannel.call(
+        guardian: user_1.guardian,
+        target_usernames: [user_1.username, user_2.username],
+      )
+
+    service_failed!(result) if result.failure?
+
+    @personal_chat_channel = result.channel
 
     [user_1, user_2].each do |u|
       Fabricate(:user_chat_channel_membership, chat_channel: public_channel, user: u)