Consider `model_settings` on `create_agent`

### Privileged issue - [x] I am a LangChain maintainer, or was asked directly by a LangChain maintainer to create an issue here. ### Issue Content See https://github.com/langchain-ai/langchain/pull/34026 as an example implementation The vast majority of model settings can be set on a model directly, ex ```py ChatOpenAI("gpt-4.1-mini", temperature=1) ``` However, there are a few settings like `strict` (for tool calling) that you must currently set via middleware's `model_settings`. We could consider adding a `model_settings` arg to `create_agent`, but this creates another redundant way to do the same thing. Right now everything you might desire is possible via middleware, but it's more verbose certainly. ### Package (Required) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-cli - [ ] 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-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general