Add shared response DTOs and enhance the response envelope middleware with
excluded paths, trace ID generation fallback, and automatic extraction of
error details from handler responses. Update default business code mapping
for 503 and 504, and adjust idempotency detection to only treat the new
envelope format as already-wrapped.
BREAKING CHANGE: responses using the old envelope format (e.g., string
`code`) are now wrapped into the new standard envelope.
