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

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Apply the SaladCloud endpoint during provider construction

When callers use the public factory as create_provider("saladcloud", None, ...), this new match selects the OpenAI-compatible branch in router.rs, but OpenAIProvider::from_config retains OpenAIConfig::default() and therefore targets https://api.openai.com/v1; the new openai_compatible_endpoint entry is not used anywhere in that construction path. Consequently, omitting the documented-optional endpoint sends requests and the SaladCloud bearer credential to OpenAI instead of SaladCloud, so the factory should apply this default endpoint or reject a missing endpoint.

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