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/AuthConfigTests.cs

@@ -0,0 +1,21 @@
+using NATS.Server;
+using NATS.Server.Auth;
+
+namespace NATS.Server.Auth.Tests;
+
+public class AuthConfigTests
+{
+    [Fact]
+    public void NatsOptions_has_auth_fields_with_defaults()
+    {
+        var opts = new NatsOptions();
+
+        opts.Username.ShouldBeNull();
+        opts.Password.ShouldBeNull();
+        opts.Authorization.ShouldBeNull();
+        opts.Users.ShouldBeNull();
+        opts.NKeys.ShouldBeNull();
+        opts.NoAuthUser.ShouldBeNull();
+        opts.AuthTimeout.ShouldBe(TimeSpan.FromSeconds(2));
+    }
+}