Conform to Open Responses

### Checked other resources - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### 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 ### Feature Description Conform to Open Responses so users don’t need to manually wrap outputs. Align LangChain’s message/streaming primitives with the Open Responses schema https://www.openresponses.org/ ### Use Case Building agents today requires bespoke “response wrapping” to normalize roles, tool results, citations, and streamed segments. Native Open Responses support in LangChain would let apps consume a single consistent schema, simplify UI rendering and tracing, and make components interchangeable across model providers without adapter code. ### Proposed Solution _No response_ ### Alternatives Considered _No response_ ### Additional Context _No response_