PERF: optimize chat user membership cleanup when removing a single user (#29833)

This optimizes a hot path when users are being removed from one or more groups since we use the `on(:user_removed_from_group)` event to call the `Chat::AutoLeaveChannels` with a `user_id` parameter. In such case, we don't need to clear the membership of all users, just that one user.

DEBUG: Also added a "-- event = <event>" comment on top of the SQL queries used by the "AutoLeaveChannels" so they show up in the logs and hopefully facilitate any performance issues that might arise in the future.

plugins/chat/app/services/chat/auto_leave_channels.rb

@@ -17,6 +17,8 @@ module Chat
       attribute :group_id, :integer
       attribute :channel_id, :integer
       attribute :category_id, :integer
+
+      validates :event, presence: true
     end
 
     step :remove_memberships
@@ -34,6 +36,7 @@ module Chat
 
       if !group_ids.include?(Group::AUTO_GROUPS[:everyone])
         sql = <<~SQL
+          -- event = #{params.event}
           DELETE FROM user_chat_channel_memberships uccm
           WHERE NOT EXISTS (
             SELECT 1
@@ -41,6 +44,7 @@ module Chat
             WHERE gu.user_id = uccm.user_id
             AND gu.group_id IN (:group_ids)
           )
+          #{params.user_id ? "AND uccm.user_id = #{params.user_id}" : ""}
           RETURNING chat_channel_id, user_id
         SQL
 
@@ -54,6 +58,7 @@ module Chat
       category_sql = params.category_id ? "AND c.id = #{params.category_id}" : ""
 
       sql = <<~SQL
+        -- event = #{params.event}
         WITH valid_permissions AS (
           SELECT gu.user_id, cg.category_id
           FROM group_users gu