astream_events v2 does not emit terminal error events for failed runs

Finding: In one external report, astream_events(version="v2") on langchain-core 1.4.8 (Windows, Python 3.12.7) emits the start event for a failing runnable and the original exception propagates, but no terminal error event such as on_chain_error is emitted (https://github.com/langchain-ai/langchain/issues/41141). The limitation: this is an external report only; KFA has not reproduced it. Reported details: The author gives a minimal offline example, a RunnableLambda that raises ValueError, consumed via astream_events with version v2 inside try/except. They state the ValueError is raised as expected but the event list lacks a terminal error event (issue body, https://github.com/langchain-ai/langchain/issues/41141). They also say the behavior was reproduced for chains/runnables, chat models, legacy LLMs and retrievers, and that failed child runs inside RunnableWithFallbacks can be left without a terminal event while the fallback continues. The author says v1 event-stream behavior is not changed and that the issue is specific to the v2 implementation. Expected behavior per the author: on_chain_error, on_chat_model_error, on_llm_error and on_retriever_error for the respective failed runs. Status: The issue was open at the time of the supplied snapshot. The only comment is from a non-maintainer (author association NONE) who says a fix with unit tests is in their fork and asks for assignment (https://github.com/langchain-ai/langchain/issues/41141#issuecomment-6051583401). No pull request, commit or release is linked, so there is no confirmed fix and no shipped release. No maintainer has confirmed the cause. The author's description of the proposed fix (adding missing error handlers and removing failed runs from the internal run map) is an unreviewed claim, not a confirmed root cause. Practical implication: If you consume v2 events to decide whether a run failed, this report suggests you should not rely on a terminal error event being present for failed runs in this version. No source-backed workaround is given, and the affected version range beyond 1.4.8 is unknown. Treat as unverified until a maintainer or independent reproduction confirms it.