feat(router): add SaladCloud as an OpenAI-compatible provider

[Maksim (mgorkii-nlplogix)]

Summary

• recognize saladcloud as an OpenAI-compatible provider kind
• expose https://ai.salad.cloud/v1 as its default endpoint
• list SaladCloud among the router's documented OpenAI-compatible providers
• add focused coverage for the provider classification and endpoint

This change is limited to the public router client. The SaladCloud rollout is intended to expose only qwen3.6-35b-a3b; canonical model-catalog and hosted-gateway configuration remain separate from this repository.

Validation

• cargo +1.97.1 test -p braintrust-llm-router — 204 unit tests and 15 integration tests passed; 1 doc test ignored
• cargo +1.97.1 clippy -p braintrust-llm-router --all-targets -- -D warnings
• cargo +1.97.1 fmt --all -- --check
• git diff --check

Documentation

• Salad AI Gateway getting started
• Salad AI Gateway model reference