Health Providers

Per-provider search health — why a provider returned nothing.

WHY THIS EXISTS: Duffel contributed zero offers to every cash search in production and there was no way to tell why from outside the box (#4838). Three very different causes are indistinguishable in the search stream:

1. credential missing — duffel_provider.search() returns [] when duffel_api_token is blank, logs a warning, and (since #3780) an empty API result is deliberately NOT a circuit-breaker failure. So a blank or misnamed env var yields zero deals, no error, no breaker trip, and no external signal at all.
2. circuit breaker open — the provider is skipped before it is called.
3. genuinely no inventory on the route — a correct, healthy answer.

All three surfaced as the same thing because the aggregator labels any zero-result provider status: "error". This endpoint separates them.

Follows the /health/auth-config convention: credentials are reported as a PRESENCE BIT only, never the value. Inherits the /health internal-auth exemption so it is curl-able without a key — it exposes no secrets, no PII and no booking state, only whether our own plumbing is connected.