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/SubListMatchBytesTests.cs

@@ -0,0 +1,16 @@
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Core.Tests;
+
+public class SubListMatchBytesTests
+{
+    [Fact]
+    public void MatchBytes_matches_subject_without_string_allocation_and_respects_remote_filter()
+    {
+        using var sl = new SubList();
+        sl.MatchBytes("orders.created"u8).PlainSubs.Length.ShouldBe(0);
+
+        sl.Insert(new Subscription { Subject = "orders.*", Sid = "1" });
+        sl.MatchBytes("orders.created"u8).PlainSubs.Length.ShouldBe(1);
+    }
+}