Core: use RFC 9457 problem details for error responses

[joostfarla]

Adopts RFC 9457 (Problem Details) for error responses and aligns the rest of the standard with the existing /rec/core/rfc9457 recommendation.

• OpenAPI: replace the legacy {code, description} exception schema with the RFC 9457 problem object and return errors as application/problem+json.
• Clause 7: rename / reword the recommendation to cover both 4xx and 5xx; add
  application/problem+json guidance and an example.

closes #1015