addChannel method
Adds a text channel to spaceId that the principals of access see with their level,
and its main thread; returns the channel ID.
Implementation
String addChannel(String spaceId, String name, Map<String, String> access) {
final id = nextId();
channels[id] = FakeChannel({
'id': id,
'space_id': spaceId,
'kind': 'text',
'name': name,
'topic': '',
'sort_key': 'a${channels.length}',
'operator': _inheritOperator(spaceId),
'read_only': false,
'version': 1,
'settings_version': 1,
'created_at': stamp(),
}, Map.of(access));
return id;
}