feat: add system event subject constants and SystemMessageHandler delegate

src/NATS.Server/Events/EventSubjects.cs

@@ -0,0 +1,49 @@
+using NATS.Server.Auth;
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Events;
+
+/// <summary>
+/// System event subject patterns.
+/// Maps to Go events.go:41-97 subject constants.
+/// </summary>
+public static class EventSubjects
+{
+    // Account-scoped events
+    public const string ConnectEvent = "$SYS.ACCOUNT.{0}.CONNECT";
+    public const string DisconnectEvent = "$SYS.ACCOUNT.{0}.DISCONNECT";
+    public const string AccountConnsNew = "$SYS.ACCOUNT.{0}.SERVER.CONNS";
+    public const string AccountConnsOld = "$SYS.SERVER.ACCOUNT.{0}.CONNS";
+
+    // Server-scoped events
+    public const string ServerStats = "$SYS.SERVER.{0}.STATSZ";
+    public const string ServerShutdown = "$SYS.SERVER.{0}.SHUTDOWN";
+    public const string ServerLameDuck = "$SYS.SERVER.{0}.LAMEDUCK";
+    public const string AuthError = "$SYS.SERVER.{0}.CLIENT.AUTH.ERR";
+    public const string AuthErrorAccount = "$SYS.ACCOUNT.CLIENT.AUTH.ERR";
+
+    // Request-reply subjects (server-specific)
+    public const string ServerReq = "$SYS.REQ.SERVER.{0}.{1}";
+
+    // Wildcard ping subjects (all servers respond)
+    public const string ServerPing = "$SYS.REQ.SERVER.PING.{0}";
+
+    // Account-scoped request subjects
+    public const string AccountReq = "$SYS.REQ.ACCOUNT.{0}.{1}";
+
+    // Inbox for responses
+    public const string InboxResponse = "$SYS._INBOX_.{0}";
+}
+
+/// <summary>
+/// Callback signature for system message handlers.
+/// Maps to Go's sysMsgHandler type in events.go:109.
+/// </summary>
+public delegate void SystemMessageHandler(
+    Subscription? sub,
+    INatsClient? client,
+    Account? account,
+    string subject,
+    string? reply,
+    ReadOnlyMemory<byte> headers,
+    ReadOnlyMemory<byte> message);