MM-56071: Thread presence indicator (#25694)

We also track the channelID of the thread opened.

Additionally on every connection create or re-connect
with an existing queue, we reset the active state to empty
to avoid any edge-cases.

https://mattermost.atlassian.net/browse/MM-56071

```release-note
NONE
```

server/channels/api4/websocket_test.go

@@ -440,6 +440,11 @@ func TestWebSocketPresence(t *testing.T) {
 	resp = <-wsClient.ResponseChannel
 	require.Nil(t, resp.Error)
 	require.Equal(t, resp.SeqReply, wsClient.Sequence-1, "bad sequence number")
+
+	wsClient.UpdateActiveThread("threadID")
+	resp = <-wsClient.ResponseChannel
+	require.Nil(t, resp.Error)
+	require.Equal(t, resp.SeqReply, wsClient.Sequence-1, "bad sequence number")
 }
 
 func TestWebSocketUpgrade(t *testing.T) {