RunnableSequence.astream() leaks TCP socket FDs on client disconnect

### 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 _No response_ ### Reproduction Steps / Example Code (Python) ```python import asyncio, multiprocessing, os, time import httpx def run_server(): import uvicorn from fastapi import FastAPI from fastapi.responses import StreamingResponse from langchain_core.prompts import ChatPromptTemplate from langchain_openai import ChatOpenAI app = FastAPI() @app.get("/health") def health(): return {"fd": len(os.listdir(f"/proc/{os.getpid()}/fd"))} @app.post("/stream") async def stream(): model = ChatOpenAI(model="gpt-4o-mini", streaming=True) prompt = ChatPromptTemplate.from_messages([ ("system", "You are helpful."), ("user", "{message}"), ]) chain = prompt | model async def generate(): async for chunk in chain.astre…