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8773cb8c4e
Adds a new `on_stream_event` hook on `LLMManagerMixin` / `AsyncCallbackHandler` that fires once per `MessagesData` event produced by `stream_v2` / `astream_v2`, with dispatch methods on `CallbackManagerForLLMRun` and `AsyncCallbackManagerForLLMRun`. This is v2's observer hook, analogous to `on_llm_new_token` in v1 but at event granularity rather than chunk. It fires uniformly whether the provider emits events natively via `_stream_chat_model_events` or goes through the chunk-to-event compat bridge — observers see the same event stream regardless of how the underlying model produces output. Primary consumer: langgraph's forthcoming `StreamProtocolMessagesHandler`, which can now be a one-line forwarder (lookup namespace metadata by run_id, push `(ns, "messages", (event, meta))` to the graph's output stream) instead of re-implementing the chunks-to-events state machine internally. Does not fire from v1 `stream()` / `astream()`. Purely additive — `on_chat_model_start`, `on_llm_end`, and `on_llm_error` continue to bracket a v2 call as they do a v1 call.