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bf64733f74
Adds `_V2StreamingCallbackHandler`, a marker class in `tracers/_streaming.py` that handlers can inherit to signal they consume `on_stream_event` rather than `on_llm_new_token`. Extracts the shared event-producing logic from `stream_v2` / `astream_v2` into `_iter_v2_events` / `_aiter_v2_events` helpers, which pick the native `_stream_chat_model_events` hook or fall back to `chunks_to_events` bridged from `_stream`. `BaseChatModel.invoke` / `ainvoke` now route through the v2 event generator when any attached handler inherits the marker: `_generate_with_cache` / `_agenerate_with_cache` gain a v2 branch, parallel to the existing v1 streaming branch, that drains the helper into a `ChatModelStream` and wraps the assembled `AIMessage` as a `ChatResult`. Caching, rate limiting, run lifecycle, and `llm_output` merging stay on the existing generate path — the v2 and v1 branches diverge only on which callback fires per chunk. The marker is a concrete class rather than a `runtime_checkable` `Protocol` on purpose: an empty Protocol matches every object and would misroute every call.