StreamMessagesHandlerV2

Intentional no-op — v1 chunks are not used on v2-flagged runs.

The v2 marker already steers invoke to the event generator, so on_llm_new_token should not fire under normal routing. This override stays a pass-through (no call to super()) to make the intent explicit and to guard against any caller (e.g. a node that calls model.stream() directly, which still fires the v1 callback) leaking AIMessageChunks onto a v2-flagged messages stream.

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.