refactor: rename remaining tests to NATS.Server.Core.Tests

- Rename tests/NATS.Server.Tests -> tests/NATS.Server.Core.Tests
- Update solution file, InternalsVisibleTo, and csproj references
- Remove JETSTREAM_INTEGRATION_MATRIX and NATS.NKeys from csproj (moved to JetStream.Tests and Auth.Tests)
- Update all namespaces from NATS.Server.Tests.* to NATS.Server.Core.Tests.*
- Replace private GetFreePort/ReadUntilAsync helpers with TestUtilities calls
- Fix stale namespace in Transport.Tests/NetworkingGoParityTests.cs

tests/NATS.Server.Core.Tests/SubList/SubListNotificationTests.cs

@@ -0,0 +1,24 @@
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Core.Tests;
+
+public class SubListNotificationTests
+{
+    [Fact]
+    public void Interest_change_notifications_are_emitted_for_local_and_remote_changes()
+    {
+        using var sl = new SubList();
+        var changes = new List<InterestChange>();
+        sl.InterestChanged += changes.Add;
+
+        var sub = new Subscription { Subject = "orders.created", Sid = "1" };
+        sl.Insert(sub);
+        sl.Remove(sub);
+        sl.ApplyRemoteSub(new RemoteSubscription("orders.*", null, "r1", "A"));
+        sl.ApplyRemoteSub(RemoteSubscription.Removal("orders.*", null, "r1", "A"));
+
+        changes.Count.ShouldBe(4);
+        changes.Select(c => c.Kind).ShouldContain(InterestChangeKind.LocalAdded);
+        changes.Select(c => c.Kind).ShouldContain(InterestChangeKind.RemoteAdded);
+    }
+}