Support Third-Party Providers Using OpenAI-Compatible Format

Key requirements:
- Accept OpenAI Chat/Completions request schema (model/messages/stream/etc.)
- Ensure compatibility with third-party providers using the OpenAI format (e.g., self-hosted gateways, multi-model routers)
- Support text, image, audio, video, and file inputs

I implemented the compatibility based on the approach used in adk-python and will submit a PR shortly.