Keep a backend whose probe path is wrong in service

A backend that 404s on the health probe path but serves queries fine was
marked down and excluded from every fan-out for good, since the fail-open
only triggers when no backend is left healthy.

Classify a probe reply that refuses the request itself - any 4xx, plus 501 -
as evidence about the probe, not the backend. Such a backend keeps serving
queries and reports the distinct probe_unsupported state on /healthz.
Transport failures and 5xx, 503 included, still mark a backend down.
Log the misconfiguration once per transition with the backend, probe path
and status. Track failure runs per outcome kind so a 404 run and a 503 run
never add up to one threshold.

server.go

@@ -676,7 +676,7 @@ type healthReport struct {
 // prober's running state for the same backend.
 type backendReport struct {
 	Reachable string `json:"reachable"` // "ok" | error text
-	State     string `json:"state"`     // healthy | unhealthy | unprobed | unmonitored
+	State     string `json:"state"`     // healthy | unhealthy | probe_unsupported | unprobed | unmonitored
 	Failures  int    `json:"consecutive_failures"`
 	Successes int    `json:"consecutive_successes"`
 	LastProbe string `json:"last_probe,omitempty"`