feat: add ClientKind enum with IsInternal extension

src/NATS.Server/ClientKind.cs

@@ -0,0 +1,22 @@
+namespace NATS.Server;
+
+/// <summary>
+/// Identifies the type of a client connection.
+/// Maps to Go's client kind constants in client.go:45-65.
+/// </summary>
+public enum ClientKind
+{
+    Client,
+    Router,
+    Gateway,
+    Leaf,
+    System,
+    JetStream,
+    Account,
+}
+
+public static class ClientKindExtensions
+{
+    public static bool IsInternal(this ClientKind kind) =>
+        kind is ClientKind.System or ClientKind.JetStream or ClientKind.Account;
+}