Add support for OCI OpenAI Responses API with Langchain/Langgraph

[furqan-shaikh-dev]

Summary

Adds support for OpenAI Responses API by introducing a new class OCIChatOpenAI. This allows users to invoke OCI Generative AI to leverage OpenAI Responses API.

Changes

• Add new file oci_generative_ai_responses_api.py which implements OCIChatOpenAI class and OCI Auth.
• Add comprehensive unit tests covering most common langchain and langgraph scenarios.
• Add examples folder to provide sample usages
• Add documentation related to the change in README.md

Testing

• Unit Tests: 5/5 tests passing
• Integration Tests: To be raised as a separate PR

Breaking Changes

None - this is a new feature that is fully backward compatible.

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