• Replace ErrorResponse with Spring's native ProblemDetail
• Extend ResponseEntityExceptionHandler and use @RestControllerAdvice
• Preserve legacy error fields and include a timestamp
• Remove manual parsing of unreadable HTTP message errors
• Update OpenAPI schemas, error response tests, and exception-handling docs

PR Description

What this PR Provides

• Describe in short sentences the goal of the PR. Use lists.
• If the PR is linked to an ADR, please provide the link.

Fixes

Review

The reviewer must double-check these points:

• The reviewer has tested the feature
• The reviewer has reviewed the implementation of the feature
• The documentation has been updated
• The feature implementation respects the Technical Doc / ADR previously produced
• The Pull Request title has a ! after the type/scope to identify the breaking
  change in the release note and ensure we will release a major version.

How to test

Initial state: application running normally, no specific data setup required.

What and how to test:

• Trigger a known business error, e.g. GET /api/v1/entity_templates/{identifier} with a non-existent identifier (404), or POST /api/v1/entity_templates with an invalid payload (400).
• Check the error response body for each error type (400, 404, 409, 500).
• Run the test suite: ./gradlew test --tests "*ApiExceptionHandlerTest*" along with the impacted controller tests (EntityControllerTest, EntityTemplateControllerTest, EntityDynamicMappingControllerTest).
• Check docs/src/static/swagger.yaml / Swagger UI to confirm error responses now reference the ProblemDetail schema.

Expected results:

• Error response bodies now follow the standard RFC 7807 ProblemDetail format: {"type": ..., "title": ..., "status": ..., "detail": ..., "instance": ...} (Content-Type application/problem+json), replacing the previous {"error": ..., "error_description": ...} format.
• HTTP status codes returned remain unchanged (404, 400, 409, 500...).

Breaking changes (if any)

• API JSON schema modification (existing resource / behavior)

Context of the Breaking Change

The API error response format has been migrated from the internal ErrorResponse class ({"error": "...", "error_description": "..."}) to Spring's RFC 7807 ProblemDetail standard (application/problem+json), for all errors (400, 404, 409, 500, etc.) across all endpoints.

Result of the Breaking Change

Clients consuming the IDP-Core API that parse error bodies on the error / error_description fields must be updated to read the new ProblemDetail format: type, title, status, detail, instance. The Content-Type of error responses changes to application/problem+json.