Function resolve_fallback_chain 

pub fn resolve_fallback_chain(
    chain: &[String],
    extra: &[(&str, &str)],
) -> Vec<String>

Resolve a [capabilities.model_catalog].fallback list (each entry an alias or an already-full slug) into full slugs, in order, via resolve_alias. An empty chain resolves to an empty Vec — the default, no-fallback-configured shape.