Provider Implementation

Checklist

1. Implement the full interface (ChatModel, Embedder, VectorStore, STT, TTS, etc.).
2. Register via init() with parent package's Register().
3. Map provider errors to core.Error with correct ErrorCode.
4. Support context cancellation.
5. Include token/usage metrics where applicable.
6. Compile-time check: var _ Interface = (*Impl)(nil).
7. Unit tests with mocked HTTP responses (httptest).

File Structure

llm/providers/openai/
├── openai.go          # Implementation + New() + init()
├── stream.go          # Streaming
├── errors.go          # Error mapping