Extending the SDK

Add a new provider by implementing translation + transport mapping layers. High-level outline (adjust to actual trait names):

1. Define capability & endpoint metadata (ModelInfo, ModelCapabilities).
2. Implement a translator that converts ChatCompletionRequest -> provider HTTP payload.
3. Implement response parser mapping provider JSON -> unified ChatCompletionResponse (and chunks for streaming).
4. Register provider config (ProviderConfigs).
5. Add optional pricing / performance metadata for routing heuristics.

Planned docs: full code template & test harness for a mock provider.