fix: make embedded NATS opt-in so tests don't hang

Move NATS startup from New() to Start(), so tests that don't use
pubsub never block on server initialization. Add a 10s timeout to
WaitForServer() and skip NATS tests gracefully when unavailable.

via.go

@@ -358,6 +358,16 @@ func (v *V) reapOrphanedContexts(suspendAfter, ttl time.Duration) {
 // Start starts the Via HTTP server and blocks until a SIGINT or SIGTERM
 // signal is received, then performs a graceful shutdown.
 func (v *V) Start() {
+	if v.pubsub == nil {
+		dn, err := getSharedNATS()
+		if err != nil {
+			v.logWarn(nil, "embedded NATS unavailable: %v", err)
+		} else {
+			v.defaultNATS = dn
+			v.pubsub = &natsRef{dn: dn}
+		}
+	}
+
 	handler := http.Handler(v.mux)
 	if v.sessionManager != nil {
 		handler = v.sessionManager.LoadAndSave(v.mux)
@@ -833,14 +843,6 @@ func New() *V {
 		v.cleanupCtx(c)
 	})
 
-	dn, err := getSharedNATS()
-	if err != nil {
-		v.logWarn(nil, "embedded NATS unavailable: %v", err)
-	} else {
-		v.defaultNATS = dn
-		v.pubsub = &natsRef{dn: dn}
-	}
-
 	return v
 }