ChatOpenRouter: max_retries=0 enables the SDK default retry policy

### 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 #40377 was automatically closed because it was submitted via CLI. This is the same report resubmitted through the required web template. ### Reproduction Steps / Example Code (Python) ```python from langchain_openrouter import ChatOpenRouter model = ChatOpenRouter( model="openai/gpt-4o-mini", api_key="unused", max_retries=0, ) print(model.client.sdk_configuration.retry_config) # Actual: Unset() # Expected: None ``` ### Error Message and Stack Trace (if applicable) ```shell ``` ### Description `ChatOpenRouter.max_retries` documents that setting the value to `0` disables retries. However, `_build_client` currently omits `retry_config` when the value is zero. OpenRouter SDK 0.10.0 interprets an omitted (`UNSET`) retry configuration as its default backoff policy, with a maximum elapsed time of 3,600,000 ms for 5xx r…