Function buildFeedSuccessResponse

Wrap a successful upstream feed result in the uniform envelope.

The original payload (typically { data, '@context' }) is preserved verbatim and augmented with the uniform contract fields:

• status is derived from items.length and warnings:
  • "unavailable" when items.length === 0 (covers the "HTTP 200 + empty array" upstream case);
  • "degraded" when items are present and any dataQualityWarnings are present (either supplied via result or passed explicitly);
  • "operational" when items are present and no warnings.
• data is normalized so that, after JSON serialization, consumers reading the backwards-compatible data field always see an array with the same contents as items (structural equality across the wire, not referential identity).
• Existing dataQualityWarnings from result are preserved and merged with any explicitly-supplied warnings (rather than clobbered). When status is "unavailable", the empty-feed reason is appended for backwards compatibility with consumers reading dataQualityWarnings.
• reason is set whenever status !== "operational".