ui : update active conversation fields in place
updateCurrentNode, applyConversationUpdate, updateConversationTimestamp
and the pin toggle replaced the whole activeConversation object, so its
identity changed on every send, tool result and rename. ChatMessages
tracks that identity to refresh sibling info, so each replacement
triggered a full refetch of every message in the conversation. Write the
changed fields instead, mirroring updateMessageAtIndex.
Assisted-by: pi:zai-org/GLM-5.3
