Add OpenAICompatAdapter that works with any OpenAI-compatible API endpoint
(OpenWebUI, vLLM, Ollama, OpenAI, Anthropic via proxy). Features:
- Async HTTP calls via httpx with configurable timeout
- Chat completions format with system + user messages
- Token usage parsing from responses
- Exponential backoff retries (configurable, default 3 attempts)
- Both streaming (SSE) and non-streaming modes
- Model listing and connection testing
- 21 tests covering construction, request building, response parsing,
retry logic, and error handling
