Do not send MDNs for hidden messages

[link2xt]

Hidden messages are marked as seen
when chat is marked as noticed.
MDNs to such messages should not be sent
as this notifies the hidden message sender
that the chat was opened.

The issue discovered by Frank Seifferth.

When chat is marked as noticed, hidden messages are marked as seen here:

core/src/chat.rs

Lines 3305 to 3322 in bf02785

	// This is to trigger emitting `MsgsNoticed` on other devices when reactions are noticed
	// locally (i.e. when the chat was opened locally).
	let hidden_messages = context
	.sql
	.query_map_vec(
	"SELECT id FROM msgs
	WHERE state=?
	AND hidden=1
	AND chat_id=?
	ORDER BY id LIMIT 100", // LIMIT to 100 in order to avoid blocking the UI too long, usually there will be less than 100 messages anyway
	(MessageState::InFresh, chat_id), // No need to check for InNoticed messages, because reactions are never InNoticed
	|row| {
	let msg_id: MsgId = row.get(0)?;
	Ok(msg_id)
	},
	)
	.await?;
	message::markseen_msgs(context, hidden_messages).await?;

[iequidoo]

Just in case, i also saw this logic, but was never thinking it's a problem because if the contact is accepted, why hide the fact we've noticed reactions from it

[link2xt]

The problem is that you can even send empty reaction message (removing reaction) and then there is no notification or anything visible, but hidden message is still added and MDN is sent when you open the chat.

There is anyway no double checkmark for hidden messages, so this is just wasting the traffic and storage and bumps "last seen" so green dot appears.

[iequidoo]

The similar problem: i opened a chat on Desktop and forgot about it. If the contact sends me a message, it immediately receives an MDN which reveals that the chat is open on my device.

Basically the problem is: you don't do anything that assumes sending MDNs to accepted contacts, but they are sent anyway and that reveals some of your state or previous actions.

[iequidoo]

Anyway, apart from other similar problems, this makes sense because marking a chat as noticed shouldn't send any MDNs to contacts. Would be good to send MDNs to self however, to remove notifications for reactions.

EDIT: MDNs to self are sent as previously. No further changes are needed.