collect_models unioned only registry.healthy() engines and re-fetched
each engine's /v1/models live. An engine loading a model is GIL-blocked
and misses the 2s health poll, so it goes "unhealthy" and ALL its models
— including a freshly-added one — drop out of the aggregated /v1/models
until the load finishes. A client (e.g. the kilo model script) polling
during a load then sees models vanish. Cache each engine's last-good
/v1/models and, when it's transiently unhealthy/unreachable, serve that
cached list instead of dropping it. The models are still assigned to the
engine and will serve once it's free.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>