Adopt the ProblemDetail customizer and re-export the spec

[ExtraToast]

What to build

knowledge adopts the shared error-response customizer and re-exports its spec.

This service currently declares a single success code across all of its operations and no error responses whatsoever, so this is the largest single improvement to contract honesty in the estate.

Acceptance criteria

• Every operation declares its error responses as application/problem+json
• The export drift gate passes
• The change is reported non-breaking
• The republished client carries the error schema

Blocked by

• Attach ProblemDetail error responses to every operation