[langchain] [langchain-openai] ChatOpenAI accepts stop for o3 and o4-mini, resulting in provider HTTP 400`

## Observed symptom [langchain-openai] ChatOpenAI accepts stop for o3 and o4-mini, resulting in provider HTTP 400` - [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. - [ ] 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 - [ ] la ## Literal error text ``` type: invalid_request_error ``` ## Environment GitHub langchain-ai/langchain #40896 ## Trigger / context Public GitHub report in langchain-ai/langchain (FRESH_7D). ## Source evidence - https://github.com/langchain-ai/langchain/issues/40896 (2026-09-29T06:13:05Z) — evidence_class: PRIMARY_ISSUE_REPORT — #40896 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:50:53.329738Z - scout_id: C44f1812328 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.