FIX: Page size edge case for null last_read_message_id (#21811)

Followup 55ef2d0698627348e4f340f079504419c0939677.

In the cases where the user has no last_read_message_id for
a channel, we want to make sure that a page_size is set for
the ChannelViewBuilder + MessagesQuery, otherwise we end up
loading way more messages than needed (the additional message
loading was fixed in the last commit).

plugins/chat/app/services/chat/channel_view_builder.rb

@@ -84,6 +84,15 @@ module Chat
     def determine_target_message_id(contract:, channel:, guardian:, **)
       if contract.fetch_from_last_read
         contract.target_message_id = channel.membership_for(guardian.user)&.last_read_message_id
+
+        # We need to force a page size here because we don't want to
+        # load all messages in the channel (since starting from 0
+        # makes them all unread). When the target_message_id is provided
+        # page size is not required since we load N messages either side of
+        # the target.
+        if contract.target_message_id.blank?
+          contract.page_size = contract.page_size || Chat::MessagesQuery::MAX_PAGE_SIZE
+        end
       end
     end