Require candidate decoders for artifact codecs

internal/core/config/effective_config_contract_test.go

@@ -427,6 +427,10 @@ func (effectiveCodec) Decode(content []byte) (effectiveArtifact, error) {
 	return value, err
 }
 
+func (codec effectiveCodec) DecodeCandidate(content []byte) (effectiveArtifact, error) {
+	return codec.Decode(content)
+}
+
 type effectiveInput struct{ key string }
 
 func (m effectiveInput) Key() string { return m.key }