[management] Say what went wrong when loading a provider's models fails

Pressing "Load models from provider" against a key the vendor refuses answered
"internal server error". Every outcome on that path is the operator's own key
or upstream, so a 500 was wrong twice over: it told them the server broke, and
it named nothing they could act on.

The failures are already typed and already have sentences written for them —
the save-time check translates the same set. Discovery now runs them through
the same classifier, so the button reports a refused credential or an
unreachable url in the words the form uses elsewhere.

ErrNoDiscovery and ErrInvalidRequest pass through untouched. The handler maps
both already, and a provider with no listing endpoint is a fact about the
catalog entry rather than a failure — the caller falls back to the catalog's
own models instead of showing an error at all.

management/internals/modules/agentnetwork/manager.go

@@ -231,7 +231,11 @@ func (m *managerImpl) DiscoverProviderModels(ctx context.Context, accountID, use
 		req.APIKey = record.APIKey
 	}
 
-	return m.modelDiscovery.Fetch(ctx, req)
+	models, err := m.modelDiscovery.Fetch(ctx, req)
+	if err != nil {
+		return nil, discoveryFailure(ctx, req.CatalogID, err)
+	}
+	return models, nil
 }
 
 // CreateProvider persists a new provider for the account. Providers have no