chat.onMessageUpdated(handler). Called when an adapter
receives a platform edit event and can provide the updated message.
Message updates are lifecycle events: they do not route through
onNewMessage, onNewMention, or onSubscribedMessage, and they are not
subject to the concurrency strategies. There is no MessageContext here
because nothing coalesces edits, every edit dispatches on its own.
The bot’s own edits are filtered out, so post-and-edit streaming does not
call this handler back once per delta.
previousMessage is the message as it read before the edit, present when
the platform sends it (Slack does). Use it to diff the change rather than
only seeing the result.