Function resolve 

pub async fn resolve(
    catalog: &dyn ModelCatalogPort,
    model: Option<&str>,
) -> ModelContext

Resolve the ModelContext for a model in one catalog round-trip.

model is None when the caller has no model to name — an agent session against an already-running server, say. That yields a passthrough context without touching the catalog, which is why every caller can hand its Option straight in rather than open-coding the empty case.

Any failure to resolve returns ModelContext::passthrough: an unknown or unreachable model costs the request its model-specific handling, never the request itself. The two failure modes are logged differently on purpose — an unknown model is routine (clients name models the catalog has never heard of), while a catalog error means something is actually broken.