Forward a protocol event from stream_v2 as a messages stream part.
Fires once per MessagesData event (message-start, per-block
content-block-*, message-finish). The transformer layer
correlates events back to a single ChatModelStream via
metadata["run_id"] — attached here so the v1
stream_mode="messages" output (which emits
(AIMessageChunk, metadata) via on_llm_new_token) keeps its
original metadata shape.
Lives on the v2 handler rather than the v1 base: content-block events are a v2-only concept, and forwarding them only when the v2 handler is attached keeps the message channel's shape predictable for v1 callers.
