Inconsistent `generations` shape in chat model streaming `on_llm_error` callbacks: emits `[[]]` and `[[chunk], []]` where every other path emits `[]`

### Submission checklist - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [x] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs #36866 ### Reproduction Steps / Example Code (Python) ```python Code: from typing import Any from langchain_core.callbacks import BaseCallbackHandler from langchain_core.language_models import FakeListChatModel from langchain_core.language_models.fake_chat_models import FakeListChatModelError from langchain_core.outputs import LLMResult class CaptureErr(BaseCallbackHandler): def __init__(self) -> None: self.response: LLMResult | None = None def on_llm_error(self, error: BaseException, **kwargs: Any) -> None: self.response = kwargs.get("response") message = "test" for i in range(len(message)): llm = FakeListChatModel(responses=[message], error_on_chunk_number=i) cb = CaptureErr() stream = llm.stream("hi", config={"callbacks": [cb]}) for _ in range(i): next(stream) try: …