[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/credentialcheck.go

@@ -49,6 +49,32 @@ func (m *managerImpl) checkProviderCredential(ctx context.Context, provider *typ
 	return status.Errorf(status.InvalidArgument, "%s", message)
 }
 
+// discoveryFailure renders a failed model listing for the operator who pressed
+// the button. Every outcome here is something they did or configured — a key
+// the vendor refused, an upstream that does not answer — so it owes them the
+// same sentence a refused save gives, not the generic 500 an unclassified
+// error turns into.
+//
+// ErrNoDiscovery and ErrInvalidRequest pass through untouched: the handler
+// already maps them, and "this provider has no listing endpoint" is a fact
+// about the catalog rather than a failure to report as one.
+func discoveryFailure(ctx context.Context, catalogID string, err error) error {
+	if errors.Is(err, modeldiscovery.ErrNoDiscovery) || errors.Is(err, modeldiscovery.ErrInvalidRequest) {
+		return err
+	}
+
+	message, _ := credentialCheckFailure(err)
+	if message == "" {
+		return err
+	}
+
+	// The operator's message carries no status code, so the vendor's number is
+	// recorded here or nowhere.
+	log.WithContext(ctx).Infof("agent network model discovery for %s failed: %v", catalogID, err)
+
+	return status.Errorf(status.InvalidArgument, "%s", message)
+}
+
 // credentialCheckFailure renders a discovery failure as the sentence the
 // provider form shows, and reports whether it should block the write.
 //