FIX: new DM channels not being published (#36799)

By setting following to "false" on new DM channels it mirrors what
happens when you hit X on a channel, that way the first message will
trigger showing the channel to the missing users

plugins/chat/app/services/chat/create_direct_message_channel.rb

@@ -99,7 +99,7 @@ module Chat
       channel.update!(optional_params) if !optional_params.empty?
     end
 
-    def create_memberships(channel:, target_users:)
+    def create_memberships(channel:, target_users:, guardian:)
       always_level = ::Chat::UserChatChannelMembership::NOTIFICATION_LEVELS[:always]
 
       memberships =
@@ -108,7 +108,7 @@ module Chat
             user_id: user.id,
             chat_channel_id: channel.id,
             muted: false,
-            following: true,
+            following: user.id == guardian.user.id,
             notification_level: always_level,
             created_at: Time.zone.now,
             updated_at: Time.zone.now,

plugins/chat/spec/services/chat/create_direct_message_channel_spec.rb

@@ -63,8 +63,9 @@ RSpec.describe Chat::CreateDirectMessageChannel do
           [current_user.id, user_1.id, user_2.id],
         )
         result.channel.user_chat_channel_memberships.each do |membership|
+          should_follow = membership.user_id == current_user.id
           expect(membership).to have_attributes(
-            following: true,
+            following: should_follow,
             muted: false,
             notification_level: "always",
           )

plugins/chat/spec/system/message_notifications_with_sidebar_spec.rb

@@ -164,6 +164,33 @@ RSpec.describe "Message notifications - with sidebar", type: :system do
         end
       end
 
+      context "when a new direct message channel is created" do
+        fab!(:other_user, :user)
+        let!(:chat_sidebar) { PageObjects::Components::Chat::Sidebar.new }
+
+        it "shows the channel in the sidebar without reload" do
+          visit("/")
+
+          expect(chat_sidebar).to have_start_new_dm
+
+          result =
+            Chat::CreateDirectMessageChannel.call(
+              guardian: other_user.guardian,
+              params: {
+                target_usernames: [current_user.username],
+              },
+            )
+          service_failed!(result) if result.failure?
+
+          dm_channel = result.channel
+
+          create_message(channel: dm_channel, creator: other_user)
+
+          expect(chat_sidebar).to have_direct_message_channel(dm_channel)
+          expect(chat_sidebar).to have_no_start_new_dm
+        end
+      end
+
       context "with dm channel" do
         fab!(:current_user, :admin)
         fab!(:user_1, :user)