bug: RePhraseQueryRetriever._aget_relevant_documents raises NotImplementedError

### 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 - [x] langchain-classic - [ ] 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 #37620 ### Reproduction Steps / Example Code (Python) ```python from langchain_core.documents import Document from langchain_core.retrievers import BaseRetriever from langchain_core.runnables import RunnableLambda from langchain_classic.retrievers.re_phraser import RePhraseQueryRetriever class FakeRetriever(BaseRetriever): def _get_relevant_documents(self, query, *, run_manager=None): return [Document(page_content=f"relevant: {query}")] async def _aget_relevant_documents(self, query, *, run_manager=None): return [Document(page_content=f"relevant: {query}")] chain = RunnableLambda(lambda q: "rephrased query") retriever = RePhraseQueryRetriever(retriever=FakeRetriever(), llm_chain=chain) # ✅ Sync works fine result = retriever.invoke("what is langchain?") print(f"Sync OK: {len(result)} doc(s)") # ❌ Asyn…