[MM-19914] Fix data races in WebSocketEvent (#13039)

* Make WebSocketEvent type immutable

* Update code to use updated immutable WebSocketEvent type

* Export WebSocketEvent fields and mark them as deprecated

api4/post_test.go

@@ -487,7 +487,7 @@ func TestCreatePostSendOutOfChannelMentions(t *testing.T) {
 	for waiting {
 		select {
 		case event := <-WebSocketClient.EventChannel:
-			require.NotEqual(t, model.WEBSOCKET_EVENT_EPHEMERAL_MESSAGE, event.Event, "should not have ephemeral message event")
+			require.NotEqual(t, model.WEBSOCKET_EVENT_EPHEMERAL_MESSAGE, event.EventType(), "should not have ephemeral message event")
 		case <-timeout:
 			waiting = false
 		}
@@ -506,12 +506,12 @@ func TestCreatePostSendOutOfChannelMentions(t *testing.T) {
 	for waiting {
 		select {
 		case event := <-WebSocketClient.EventChannel:
-			if event.Event != model.WEBSOCKET_EVENT_EPHEMERAL_MESSAGE {
+			if event.EventType() != model.WEBSOCKET_EVENT_EPHEMERAL_MESSAGE {
 				// Ignore any other events
 				continue
 			}
 
-			wpost := model.PostFromJson(strings.NewReader(event.Data["post"].(string)))
+			wpost := model.PostFromJson(strings.NewReader(event.GetData()["post"].(string)))
 
 			acm, ok := wpost.Props[model.PROPS_ADD_CHANNEL_MEMBER].(map[string]interface{})
 			require.True(t, ok, "should have received ephemeral post with 'add_channel_member' in props")