test(jackson2): cover concrete throwable problem deserialization

[codingkiddo]

Summary

This clarifies how to deserialize directly into a concrete custom subtype of AbstractThrowableProblem with Jackson 2.

When Jackson reads such a subtype directly, the problem type URI may otherwise be interpreted as a polymorphic subtype id, which can lead to an InvalidTypeIdException.

Changes

• Add a Jackson 2 test covering direct deserialization of a concrete AbstractThrowableProblem subtype
• Add a test fixture demonstrating the use of @JsonTypeInfo(use = JsonTypeInfo.Id.NONE)
• Document this pattern in the README for custom concrete subtype deserialization

Verification

mvn clean verify

Related

Relates to #485