FIX: Make sure marking channels read propagates to all tabs (#20802)

Instead of just marking the state read in JS for each channel
after the AJAX call, we can instead just rely on the MessageBus
user-tracking-state chat channel, and publish the state to all
the channels affected in MarkAllUserChannelsRead. This will make
it so the blue dots for the channels are cleared across all tabs.

plugins/chat/spec/services/chat/mark_all_user_channels_read_spec.rb

@@ -128,6 +128,13 @@ RSpec.describe Chat::MarkAllUserChannelsRead do
           ).count
         }.by(-2)
       end
+
+      it "publishes tracking state for all affected channels" do
+        messages = MessageBus.track_publish { result }
+        expect(
+          messages.select { |m| m.channel == "/chat/user-tracking-state/#{current_user.id}" }.count,
+        ).to eq(3)
+      end
     end
   end
 end