pinMessage method

@override Future<ChatResult<void>> pinMessage(String roomId, String messageId)

Pins a message in a room so it appears in the pinned list.

Visible to all members (not a per-user preference, unlike ChatRoomsApi.pin). Admin/owner gated by default — backends may loosen this via room config. Backend emits MessagePinnedEvent so other members see the pin in real time.

Implementation

@override
Future<ChatResult<void>> pinMessage(String roomId, String messageId) async =>
    const ChatSuccess(null);