Bug: @lru_cache-ed async httpx client causes APIConnectionError across event loops

### 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) - [ ] langchain - [x] 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 - [ ] 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 import threading from concurrent.futures import ThreadPoolExecutor, as_completed from langchain_openai import ChatOpenAI llm = ChatOpenAI(model="gpt-4o-mini", temperature=0) errors = [] successes = [] lock = threading.Lock() def worker(thread_id: int): """Each thread runs asyncio.run() which creates a fresh event loop.""" for cycle in range(5): async def call(): return await llm.ainvoke(f"Reply with only: t{thread_id}c{cycle}") try: result = asyncio.run(call()) with lock: successes.append((thread_id, cycle)) except Exception as e: with lock: errors.append((thread_id, cycle, e)) print(f"Thread {thread_id} cycle …