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/Auth/AuthModelAndCalloutConstantsParityTests.cs

@@ -0,0 +1,46 @@
+using NATS.Server.Auth;
+
+namespace NATS.Server.Auth.Tests.Auth;
+
+public class AuthModelAndCalloutConstantsParityTests
+{
+    [Fact]
+    public void NkeyUser_exposes_parity_fields()
+    {
+        var now = DateTimeOffset.UtcNow;
+        var nkeyUser = new NKeyUser
+        {
+            Nkey = "UABC",
+            Issued = now,
+            AllowedConnectionTypes = new HashSet<string> { "STANDARD", "WEBSOCKET" },
+            ProxyRequired = true,
+        };
+
+        nkeyUser.Issued.ShouldBe(now);
+        nkeyUser.ProxyRequired.ShouldBeTrue();
+        nkeyUser.AllowedConnectionTypes.ShouldContain("STANDARD");
+    }
+
+    [Fact]
+    public void User_exposes_parity_fields()
+    {
+        var user = new User
+        {
+            Username = "alice",
+            Password = "secret",
+            AllowedConnectionTypes = new HashSet<string> { "STANDARD" },
+            ProxyRequired = false,
+        };
+
+        user.ProxyRequired.ShouldBeFalse();
+        user.AllowedConnectionTypes.ShouldContain("STANDARD");
+    }
+
+    [Fact]
+    public void External_auth_callout_constants_match_go_subjects_and_header()
+    {
+        ExternalAuthCalloutAuthenticator.AuthCalloutSubject.ShouldBe("$SYS.REQ.USER.AUTH");
+        ExternalAuthCalloutAuthenticator.AuthRequestSubject.ShouldBe("nats-authorization-request");
+        ExternalAuthCalloutAuthenticator.AuthRequestXKeyHeader.ShouldBe("Nats-Server-Xkey");
+    }
+}