MM-59854 Fully allow at mentions in message attachment field values and add E2E tests (#28018)

* Add descriptions to test cases for getNeededAtMentionedUsernames

* MM-59854 Load users who are at-mentioned in message attachment fields

* MM-59854 Add server support for at-mentioning users in message attachment fields

* Migrate support/external_commands from JS to TS

* MM-59584 Ensure that at-mentions never show in other message attachment fields

* MM-59584 Add E2E tests for how we load users based on at mentions

* Use new E2E test helpers in other places

* Update snapshots

server/channels/app/notification.go

@@ -1410,6 +1410,12 @@ func getMentionsEnabledFields(post *model.Post) model.StringArray {
 		if attachment.Text != "" {
 			ret = append(ret, attachment.Text)
 		}
+
+		for _, field := range attachment.Fields {
+			if valueString, ok := field.Value.(string); ok && valueString != "" {
+				ret = append(ret, valueString)
+			}
+		}
 	}
 	return ret
 }