feat: add Account type with per-account SubList and client tracking

tests/NATS.Server.Tests/AccountTests.cs

@@ -0,0 +1,35 @@
+using NATS.Server.Auth;
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Tests;
+
+public class AccountTests
+{
+    [Fact]
+    public void Account_has_name_and_own_sublist()
+    {
+        var account = new Account("test-account");
+
+        account.Name.ShouldBe("test-account");
+        account.SubList.ShouldNotBeNull();
+        account.SubList.Count.ShouldBe(0u);
+    }
+
+    [Fact]
+    public void Account_tracks_clients()
+    {
+        var account = new Account("test");
+
+        account.ClientCount.ShouldBe(0);
+        account.AddClient(1);
+        account.ClientCount.ShouldBe(1);
+        account.RemoveClient(1);
+        account.ClientCount.ShouldBe(0);
+    }
+
+    [Fact]
+    public void GlobalAccount_has_default_name()
+    {
+        Account.GlobalAccountName.ShouldBe("$G");
+    }
+}