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
Define the LLM adapter interface in backend/engine/adapters/base.py with
async methods complete(), list_models(), and test_connection(). The
AdapterResponse dataclass holds response text, token counts, latency,
model name, and raw metadata. Includes 11 tests covering instantiation
guards, concrete subclass behavior, and dataclass semantics.
