Human-readable reason describing why the feed is empty
(also surfaced in dataQualityWarnings for backwards
compatibility with consumers reading that field).
Optionalmeta: FeedErrorMetaOptional machine-readable failure metadata. When provided,
errorCode, retryable, and upstream are included in the
response envelope so downstream consumers can classify the
failure and decide whether to retry, fall back, or skip.
Build an empty feed response under the uniform contract.
Returns the same envelope shape as buildFeedSuccessResponse with
status: "unavailable"anditems: []. This helper is used when the upstream returned 404 / empty body / error-in-body and we have no fresh data to report."degraded"is intentionally not accepted here because it denotes "partial data with warnings" — for that case, call buildFeedSuccessResponse with the partial payload and the warnings (statuswill be derived as"degraded").