[Bug]: ChatOpenRouter leaks httpx.AsyncClient instances causing Ephemeral Port Exhaustion

### 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 _No response_ ### Reproduction Steps / Example Code (Python) ```python import asyncio from langchain_openrouter import ChatOpenRouter import psutil import os async def reproduce_leak(): # Simulating high-concurrency or dynamic dynamic instantiation of ChatOpenRouter for _ in range(100): # Due to default app_url and app_title, extra_headers are always populated, # forcing _build_client() to create new httpx.Client and httpx.AsyncClient instances. llm = ChatOpenRouter(model="anthropic/claude-3-haiku") # llm is used here... # When llm goes out of scope, there is no .aclose() or destructor to clean up # the underlying httpx.AsyncClient. # Observe the number of unclosed connections/file descriptors process = psutil.Process(os.getpid()) print(f…