Function discover_model_capabilities 

pub async fn discover_model_capabilities(
    cache: &ModelCapabilitiesCache,
    provider: &str,
    model_id: &str,
    api_key: Option<&str>,
    base_url: Option<&str>,
) -> Option<ModelCapabilities>

Discover model capabilities from provider API and store in cache.

Returns the discovered capabilities, or None if discovery failed. Failures are logged but don’t prevent operation - we fall back to heuristics.