chore: fix replicasync to function for NATS even if no DERP relay address is set (#27235)

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Fixes `replicasync` so that it keeps peers even if they don't set a RelayAddress. This allows NATS to function even if you are not running a DERP relay on the primay Coderd instances.

Also renames some replicasync functions to make it clear they are for DERP.

enterprise/coderd/coderd.go

@@ -1024,7 +1024,7 @@ func (api *API) updateEntitlements(ctx context.Context) error {
 					// Only update DERP mesh if the built-in server is enabled.
 					if api.Options.DeploymentValues.DERP.Server.Enable {
 						addresses := make([]string, 0)
-						for _, replica := range api.replicaManager.Regional() {
+						for _, replica := range api.replicaManager.DERPReplicasThisRegion() {
 							// Don't add replicas with an empty relay address.
 							if replica.RelayAddress == "" {
 								continue

enterprise/coderd/coderd_test.go

@@ -671,10 +671,7 @@ func TestMultiReplica_NATSPubsubPeers(t *testing.T) {
 	t.Cleanup(func() { _ = natsB.Close() })
 
 	mgr, err := replicasync.New(ctx, logger.Named("replica-b"), db, pgPubsub, &replicasync.Options{
-		ID: uuid.New(),
-		// port doesn't matter because we don't have an API up, but replicasync will refuse peers that don't set
-		// RelayAddress at all.
-		RelayAddress:   "https://127.0.0.1",
+		ID:             uuid.New(),
 		ClusterHost:    "127.0.0.1",
 		RegionID:       12345,
 		UpdateInterval: testutil.IntervalFast,

enterprise/coderd/workspaceproxy.go

@@ -723,7 +723,7 @@ func (api *API) workspaceProxyRegister(rw http.ResponseWriter, r *http.Request)
 	}
 
 	// Find sibling regions to respond with for derpmesh.
-	siblings := api.replicaManager.InRegion(regionID)
+	siblings := api.replicaManager.DERPReplicasInRegion(regionID)
 	siblingsRes := make([]codersdk.Replica, 0, len(siblings))
 	for _, replica := range siblings {
 		if replica.ID == req.ReplicaID {