[langchain-openai] with_structured_output() silently drops previously bound tools and lacks support for OpenAI native tool bindings

### Checked other resources - [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 - [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 [Related issue: #28848](https://github.com/langchain-ai/langchain/issues/28848) This is related to #28848 (`bind_tools` not callable after `with_structured_output`), which describes the reverse ordering problem. This issue covers a different but connected scenario: when `.bind(tools=...)` is called **before** `.with_structured_output()`, the tools are **silently dropped** from the API request with no error or warning. Together, these issues show that `with_structured_output()` and tool bindings are fundamentally incompatible in **both** directions — and neither direction gives the user a clear signal. ### Reproduction Steps / Example Code (Python) ```python from langchain_openai import ChatOpenAI from langchain_core.utils.function_calling import convert_to_openai_function from pydantic import BaseModel, Field class Weath…