JsonError._embedded serialization broken with micronaut 3.4.0

[yawkat]

Issue description

In 3.4.0, the Resource class became @Introspected. Before this change, the OptionalMultiValuesSerializer would fall back on the ObjectSerializer, which in turn would use a RuntimeTypeSerializer which would delegate to the right subtype for serialization.

However after the change, ObjectSerializer will simply specialize to the serializer for Resource, which will ignore properties of subtypes of Resource (e.g. the message of a JsonError). This manifests in the test read / write JsonError test case failing.

Not sure how to fix this. It seems to me like the addition of @Introspected should not change serialization behavior this way.