langchain-openrouter: ChatOpenRouter creates fresh httpx clients per instantiation (no default-client caching)

### 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 - [ ] 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 - [x] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs Same class of bug as #32489 / PR #32531 (AzureChatOpenAI). Compounded by openai/openai-python#763 (streaming connections not returned to pool). ### Reproduction Steps / Example Code (Python) ```python import gc, os, asyncio from langchain_openrouter import ChatOpenRouter os.environ.setdefault("OPENROUTER_API_KEY", "sk-or-...") # any valid key def count_sockets(): import psutil p = psutil.Process() return sum(1 for c in p.net_connections(kind="tcp") if c.status == "ESTABLISHED") async def main(): print("before:", count_sockets()) for i in range(20): m = ChatOpenRouter(model="openai/gpt-4o-mini") await m.ainvoke("hi") del m gc.collect() print("after 20 fresh instances:", count_sockets()) asyncio.run(main()) ``` ### Error Message and Stack…