refactor: extract NATS.Server.Auth.Tests project

Move 50 auth/accounts/permissions/JWT/NKey test files from
NATS.Server.Tests into a dedicated NATS.Server.Auth.Tests project.
Update namespaces, replace private GetFreePort/ReadUntilAsync helpers
with TestUtilities calls, replace Task.Delay with TaskCompletionSource
in test doubles, and add InternalsVisibleTo.

690 tests pass.

tests/NATS.Server.Auth.Tests/AccountTests.cs

@@ -0,0 +1,71 @@
+using NATS.Server.Auth;
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Auth.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");
+    }
+
+    [Fact]
+    public void Account_enforces_max_connections()
+    {
+        var acc = new Account("test") { MaxConnections = 2 };
+        acc.AddClient(1).ShouldBeTrue();
+        acc.AddClient(2).ShouldBeTrue();
+        acc.AddClient(3).ShouldBeFalse(); // exceeds limit
+        acc.ClientCount.ShouldBe(2);
+    }
+
+    [Fact]
+    public void Account_unlimited_connections_when_zero()
+    {
+        var acc = new Account("test") { MaxConnections = 0 };
+        acc.AddClient(1).ShouldBeTrue();
+        acc.AddClient(2).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Account_enforces_max_subscriptions()
+    {
+        var acc = new Account("test") { MaxSubscriptions = 2 };
+        acc.IncrementSubscriptions().ShouldBeTrue();
+        acc.IncrementSubscriptions().ShouldBeTrue();
+        acc.IncrementSubscriptions().ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Account_decrement_subscriptions()
+    {
+        var acc = new Account("test") { MaxSubscriptions = 1 };
+        acc.IncrementSubscriptions().ShouldBeTrue();
+        acc.DecrementSubscriptions();
+        acc.IncrementSubscriptions().ShouldBeTrue(); // slot freed
+    }
+}