TypeError in merge_lists when streaming Mistral responses with inline citations

### Checked other resources - [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) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [x] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs _No response_ ### Reproduction Steps / Example Code (Python) ```python from langchain_core.utils._merge import merge_lists merged = [ "start answer...", {"type": "reference", "index": 0, "reference_ids": ["iKcb2CAQ7"]}, "other answer..." ] # Another reference to merge (from streaming chunk) other = [{"type": "reference", "index": 0, "reference_ids": ["DGqzzmqc3"]}] # This raises TypeError result = merge_lists(merged, other) ``` ### Error Message and Stack Trace (if applicable) ```shell File ".../langchain_core/utils/_merge.py", line 114, in <listcomp> if "index" in e_left and e_left["index"] == e["index"] ~~~~~~~^^^^^^^^^ TypeError: string indices must be integers, not 'str' ``` ### Description Model: mistral-medium-2505 (but likely affects any Mistral model returning citations) The error occu…