Adopt the customizer and annotate the domain error codes

[ExtraToast]

What to build

agents-api adopts the shared error-response customizer and annotates the domain-specific failure codes it actually returns.

This service already declares a handful of error responses and already emits the canonical error schemas, so the customizer fills in the universal codes while the operations that genuinely return conflict, not-found and unavailable responses declare them explicitly. The result is a spec that neither under-promises nor over-promises.

Acceptance criteria

• Universal error responses are present on every operation
• Operations that return domain-specific codes declare exactly those codes
• No operation declares a code it cannot return
• Export drift passes and the change is reported non-breaking

Blocked by

• Attach ProblemDetail error responses to every operation