feature/conn25: return unavailable from connector PeerAPI when conn25

not configured

We want all hooks and handlers to do as little as possible if the node
is not configured for conn25 or has no configured apps.

Updates tailscale/corp#39033

Signed-off-by: Michael Ben-Ami <mzb@tailscale.com>

feature/conn25/conn25.go

@@ -100,6 +100,9 @@ func handleConnectorTransitIP(h ipnlocal.PeerAPIHandler, w http.ResponseWriter,
 		http.Error(w, "miswired", http.StatusInternalServerError)
 		return
 	}
+	if !e.conn25.isConfigured() {
+		http.Error(w, "conn25 not configured", http.StatusServiceUnavailable)
+	}
 	e.handleConnectorTransitIP(h, w, r)
 }