refactor(embeddednats): ServerConfig with optional TLS

Collapses Start/StartHost/StartHostAuth onto StartServer(ServerConfig) so
auth and a TLS config can be set without growing the parameter list further.
When TLS is set the server presents the certificate and requires TLS on the
data plane; the wrappers preserve the existing no-auth/no-TLS behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/embeddednats/embeddednats.go

@@ -6,22 +6,33 @@
 package embeddednats
 
 import (
+	"crypto/tls"
 	"fmt"
 	"time"
 
 	server "github.com/nats-io/nats-server/v2/server"
 )
 
-// Start launches an embedded nats-server with JetStream enabled, listening on
-// the given port and persisting JetStream state under storeDir. The listen host
-// is left at the nats-server default ("0.0.0.0", all interfaces). It blocks
-// until the server is ready to accept connections (up to 5s) and returns the
-// running server. The caller is responsible for calling Shutdown on it.
-//
-// Start is a thin backward-compatible wrapper over StartHost; callers that need
-// to control the bind interface (loopback vs LAN) should use StartHost directly.
+// ServerConfig is the full set of knobs for the embedded NATS server. The zero
+// value (empty StoreDir aside) yields a dev-friendly server: JetStream on, bound
+// to all interfaces, no client auth, no TLS. Secured deployments set Auth and
+// TLS; tests set Host to loopback and a free Port.
+type ServerConfig struct {
+	StoreDir string // JetStream store directory
+	Host     string // bind interface; "" = nats-server default ("0.0.0.0")
+	Port     int    // listen port
+	// Auth, when non-nil, is installed as CustomClientAuthentication so the data
+	// plane only accepts approved clients (nkey signature + bus allowlist).
+	Auth server.Authentication
+	// TLS, when non-nil, makes the server present a certificate and require TLS
+	// on the data plane. Clients must trust the issuing CA (see busauth).
+	TLS *tls.Config
+}
+
+// Start is a thin backward-compatible wrapper: embedded JetStream server on the
+// default interface, no auth, no TLS.
 func Start(storeDir string, port int) (*server.Server, error) {
-	return StartHost(storeDir, "", port)
+	return StartServer(ServerConfig{StoreDir: storeDir, Port: port})
 }
 
 // StartHost is Start with explicit control over the bind interface. host selects
@@ -30,34 +41,42 @@ func Start(storeDir string, port int) (*server.Server, error) {
 // to expose it to the LAN so remote peers (phones, other PCs) can connect. An
 // empty host falls back to the nats-server default ("0.0.0.0", all interfaces).
 func StartHost(storeDir, host string, port int) (*server.Server, error) {
-	return StartHostAuth(storeDir, host, port, nil)
+	return StartServer(ServerConfig{StoreDir: storeDir, Host: host, Port: port})
 }
 
 // StartHostAuth is StartHost with an optional custom client authenticator. When
-// auth is non-nil it is installed as Options.CustomClientAuthentication, so the
-// data plane only accepts clients the authenticator approves (nkey signature +
-// bus allowlist). When auth is nil the server accepts any client (the legacy,
-// network-trusted behavior) — used by dev stacks and tests that have not enabled
-// bus auth.
+// auth is non-nil only clients the authenticator approves may connect; when nil
+// the server accepts any client (legacy, network-trusted behavior).
 func StartHostAuth(storeDir, host string, port int, auth server.Authentication) (*server.Server, error) {
+	return StartServer(ServerConfig{StoreDir: storeDir, Host: host, Port: port, Auth: auth})
+}
+
+// StartServer launches an embedded nats-server with JetStream from cfg. It
+// blocks until the server is ready to accept connections (up to 5s) and returns
+// the running server; the caller must Shutdown it.
+func StartServer(cfg ServerConfig) (*server.Server, error) {
 	opts := &server.Options{
 		JetStream:  true,
-		StoreDir:   storeDir,
-		Host:       host,
-		Port:       port,
+		StoreDir:   cfg.StoreDir,
+		Host:       cfg.Host,
+		Port:       cfg.Port,
 		DontListen: false,
 		// Keep the embedded server quiet by default; the host app logs the URLs.
 		NoLog:  true,
 		NoSigs: true,
 	}
-	if auth != nil {
-		opts.CustomClientAuthentication = auth
+	if cfg.Auth != nil {
+		opts.CustomClientAuthentication = cfg.Auth
 		// A CustomClientAuthentication alone does not make the server advertise a
 		// nonce in its INFO line, and nats.go refuses to connect with an nkey to a
 		// server that does not ("nkeys not supported by the server"). Forcing the
 		// nonce makes nkey clients sign the challenge our authenticator verifies.
 		opts.AlwaysEnableNonce = true
 	}
+	if cfg.TLS != nil {
+		opts.TLSConfig = cfg.TLS
+		opts.TLS = true
+	}
 
 	ns, err := server.NewServer(opts)
 	if err != nil {