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/AccountStatsTests.cs

@@ -0,0 +1,48 @@
+using NATS.Server.Auth;
+
+namespace NATS.Server.Auth.Tests;
+
+public class AccountStatsTests
+{
+    [Fact]
+    public void Account_tracks_inbound_stats()
+    {
+        var account = new Account("test");
+        account.IncrementInbound(1, 100);
+        account.IncrementInbound(1, 200);
+        account.InMsgs.ShouldBe(2);
+        account.InBytes.ShouldBe(300);
+    }
+
+    [Fact]
+    public void Account_tracks_outbound_stats()
+    {
+        var account = new Account("test");
+        account.IncrementOutbound(1, 50);
+        account.IncrementOutbound(1, 75);
+        account.OutMsgs.ShouldBe(2);
+        account.OutBytes.ShouldBe(125);
+    }
+
+    [Fact]
+    public void Account_stats_start_at_zero()
+    {
+        var account = new Account("test");
+        account.InMsgs.ShouldBe(0);
+        account.OutMsgs.ShouldBe(0);
+        account.InBytes.ShouldBe(0);
+        account.OutBytes.ShouldBe(0);
+    }
+
+    [Fact]
+    public void Account_stats_are_independent()
+    {
+        var account = new Account("test");
+        account.IncrementInbound(5, 500);
+        account.IncrementOutbound(3, 300);
+        account.InMsgs.ShouldBe(5);
+        account.OutMsgs.ShouldBe(3);
+        account.InBytes.ShouldBe(500);
+        account.OutBytes.ShouldBe(300);
+    }
+}