Set a conversation remark

Conversation remarks are a Commercial extension. The current OpenIMConversationItem and OpenIMSetConversationParams have no independent remark field, so this plugin release cannot safely write that capability.

Do not encode a remark into ex and present it as a standard Core field. Maintain it through an authoritative commercial business API and merge the returned business data into the conversation UI.

If a future contract adds the field, the interface/schema hashes will require this page to be reviewed again. Until then, neither public nor commercial clients should call a setter that does not exist.