Add SiliconFlow (硅基流动) as an OpenAI-compatible gateway provider.
SiliconFlow hosts multiple models (Qwen, DeepSeek, etc.) via an
OpenAI-compatible API at https://api.siliconflow.cn/v1.
Changes:
- Add ProviderSpec for siliconflow in providers/registry.py
- Add siliconflow field to ProvidersConfig in config/schema.py
Co-authored-by: Cursor <cursoragent@cursor.com>
