fix(observability): LegacyUnhealthy must not fire while up{job=legacy-api} is absent (WP-15 supplies it)

vm/observability/grafana/provisioning/alerting/rules.yaml

@@ -157,7 +157,8 @@ groups:
       model:
         refId: A
         instant: true
-        expr: (count(up{job="legacy-api"} == 1) or vector(0)) < bool 2
+        expr: count(up{job="legacy-api"} == 1) < bool 2
+        # no series until WP-15 ships Alloy on the legacy VMs: absent -> NoData -> OK (the `or vector(0)` form fired continuously)
     - refId: C
       datasourceUid: __expr__
       model: