Function fetch_models 

pub async fn fetch_models(
    provider_id: &str,
    api_key: Option<&str>,
    base_url_override: Option<&str>,
) -> Result<Vec<String>, String>

Fetch the list of available models from a provider’s API.

Returns Err with a human-readable message on any failure — no silent fallbacks. Callers should display the error to the user.