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.

Start by reading the adk-python compatibility approach mentioned in the issue, then locate the Go API entry points for chat and completions requests. Done means accepting the stated OpenAI request fields and supporting text, image, audio, video, and file inputs for third-party providers; no Go files or tests are named yet.

Written by the indexing model from the issue text.