StreamTransformerKeep internal model calls out of run.messages and the raw event log.
Used by middleware that makes internal model calls and runs before built-in transformers.
For tagged events, streamed message-start events are marked as tool-role and
whole-AIMessage payloads are cleared so MessagesTransformer ignores them.
The mutated events are then dropped from the raw log.
Only events within this transformer's scope are modified.