Get Connector Health Endpoint

Drill-down view: every deployment for one connector + recent events.

When the connector has health rows, returns them plus the current coverage verdict so stale/superseded rows cannot mask a missing current deployment. When it has none, we no longer collapse straight to 404 (ARD-1247): if the connector exists AND is in a live apply state, it is EXPECTED to be observed, so we return an explicit not_observed payload (200) instead. Only a connector that does not exist, or is not in a streaming state (no coverage story to tell), still 404s.