DEV: enforces system user membership (#23771)

System user will automatically adds itself to the channel when trying to send a message.

plugins/chat/app/services/chat/create_message.rb

@@ -24,6 +24,7 @@ module Chat
     model :channel
     policy :allowed_to_join_channel
     policy :allowed_to_create_message_in_channel, class_name: Chat::Channel::MessageCreationPolicy
+    step :enforce_system_membership
     model :channel_membership
     model :reply, optional: true
     policy :ensure_reply_consistency
@@ -72,6 +73,10 @@ module Chat
       guardian.can_join_chat_channel?(channel)
     end
 
+    def enforce_system_membership(guardian:, channel:, **)
+      channel.add(guardian.user) if guardian.user.is_system_user?
+    end
+
     def fetch_channel_membership(guardian:, channel:, **)
       Chat::ChannelMembershipManager.new(channel).find_for_user(guardian.user)
     end

plugins/chat/spec/services/chat/create_message_spec.rb

@@ -183,6 +183,12 @@ RSpec.describe Chat::CreateMessage do
             it { is_expected.to fail_a_policy(:allowed_to_join_channel) }
           end
 
+          context "when user is system" do
+            fab!(:user) { Discourse.system_user }
+
+            it { is_expected.to be_a_success }
+          end
+
           context "when user can join channel" do
             before { user.groups << Group.find(Group::AUTO_GROUPS[:trust_level_1]) }