create_agent with ProviderStrategy binds empty tools list and breaks OpenAI-compatible providers

### 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) - [x] 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 With vLLM `v0.20.0+`, this consistently fails with an error equivalent to: ```text `tools` must not be an empty array. Either provide at least one tool or omit the field entirely. ``` Related provider-side issue: - vllm-project/vllm#39741 ### Reproduction Steps / Example Code (Python) ```python from pydantic import BaseModel from langchain.agents import create_agent from langchain_openai import ChatOpenAI class MySchema(BaseModel): answer: str model = ChatOpenAI( model="...", base_url="http://openai-compatible-provider/v1", api_key="test", ) agent = create_agent( model=model, response_format=MySchema, ) agent.invoke( {"messages": [{"role": "user", "content": "hello"}]} ) ``` ### Error Message and Stack Trace (if applicable) ```shell {'error': ValueError('`tools` must not be an empty array. Either…