Add SiliconFlow providers

[123xiaode456-boop]

Summary

• Add SiliconFlow CN and international SiliconFlow provider configs.
• Register both providers in the Catwalk embedded provider list.
• Add coverage to ensure both SiliconFlow providers are present, OpenAI-compatible, and have valid default models.

Details

This adds two independent SiliconFlow API platforms:

• siliconflow-cn: https://api.siliconflow.cn/v1, API key $SILICONFLOW_CN_API_KEY
• siliconflow: https://api.siliconflow.com/v1, API key $SILICONFLOW_API_KEY

Both providers use OpenAI-compatible APIs and include current default model choices for large and small model slots.

Test

go test ./internal/providers -run TestSiliconFlowProvidersRegistered -count=1
go test ./internal/providers -count=1
go test ./... -count=1

Related

SiliconFlow task: siliconflow/siliconflow-open-source-tasks#3