Function create_model_provider_for_alias 

pub fn create_model_provider_for_alias(
    config: &Config,
    family: &str,
    alias: &str,
    api_key: Option<&str>,
    options: &ModelProviderRuntimeOptions,
) -> Result<Box<dyn ModelProvider>, Error>

Factory: create model_provider with full alias context.

(config, family, alias) lets each family branch read its own typed alias config (config.providers.models.<family>.get(alias)) directly — no flat per-family extras dumping ground. Production callers with agent context (delegate, llm_task, model routing, gateway) use this.