onMessageDrop

Called when the SDK notices the server did not deliver every live message on the topic. PubSubMessageDropInfo.droppedCount is coalesced, so one callback can stand for many messages.

Two things produce it: the server reports a range it discarded because this subscriber fell behind (PubSubSubscribeOptions.maxQueue), or delivery resumes after a shed and the SDK sees the message sequence jump.

Treat it as a hint that something was missed, not a complete loss signal. A loss is only detectable once delivery resumes, so messages shed at the very end of a burst are never reported — and under RealtimeOverflow.DROP the server tracks no sequence numbers at all, so this never fires.