FIX: Make sure "last visit" is not shown after marking all messages read (#20782)

Followup to a0381157e9, we just
need to make sure we set currentUserMembership.last_read_message_id
to the last_read_message_id from the updated memberships after marking
all channels read, otherwise we do not scroll to the bottom and still
show the "last visit" separators in channels that have been
marked read.

plugins/chat/spec/system/shortcuts/mark_all_read_spec.rb

@@ -6,7 +6,8 @@ RSpec.describe "Shortcuts | mark all read", type: :system, js: true do
   fab!(:channel_2) { Fabricate(:chat_channel) }
   fab!(:channel_3) { Fabricate(:chat_channel) }
 
-  let(:chat_page) { PageObjects::Pages::Chat.new }
+  let(:chat_sidebar) { PageObjects::Pages::Sidebar.new }
+  let(:channel_page) { PageObjects::Pages::ChatChannel.new }
   let(:drawer) { PageObjects::Pages::ChatDrawer.new }
 
   before do
@@ -15,8 +16,9 @@ RSpec.describe "Shortcuts | mark all read", type: :system, js: true do
     sign_in(user_1)
     Fabricate(:chat_message, chat_channel: channel_1)
     Fabricate(:chat_message, chat_channel: channel_1)
-    Fabricate(:chat_message, chat_channel: channel_2)
-    Fabricate(:chat_message, chat_channel: channel_2)
+    10.times do |i|
+      Fabricate(:chat_message, chat_channel: channel_2, message: "all read message #{i}")
+    end
   end
 
   context "when chat is open" do
@@ -37,6 +39,9 @@ RSpec.describe "Shortcuts | mark all read", type: :system, js: true do
         expect(page).not_to have_css(
           ".sidebar-section-link.channel-#{channel_2.id} .sidebar-section-link-suffix.unread",
         )
+        chat_sidebar.open_channel(channel_2)
+        expect(page).to have_content("all read message 9")
+        expect(page).not_to have_content(I18n.t("js.chat.last_visit"))
       end
     end
   end