FIX: correctly handles navigating to a message (#19614)

Recent changes surfaced the various issues with this codepath:
- we were not correctly reseting `messageLookup` leading to us trying to scroll to a non existing message in the view
- we were calling markAsRead which would scroll to the bottom, even if we had a target message
- we were not debouncing fetchMessages, which could cause multiple reload of the messages when loading it with a targetMessageId: first fetch from last read and then immediately fetch from targetMessageId
- other naming inconsistencies
- not handling drawer

This commit also adds tests for classic scenarios related to this use case.

plugins/chat/spec/system/page_objects/chat/chat.rb

@@ -3,6 +3,10 @@
 module PageObjects
   module Pages
     class Chat < PageObjects::Pages::Base
+      def prefers_full_page
+        page.execute_script("window.localStorage.setItem('discourse_chat_preferred_mode', '\"FULL_PAGE_CHAT\"');")
+      end
+
       def open_from_header
         find(".chat-header-icon").click
       end