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/ClientKindCommandMatrixTests.cs

@@ -0,0 +1,14 @@
+using NATS.Server.Protocol;
+
+namespace NATS.Server.Core.Tests;
+
+public class ClientKindCommandMatrixTests
+{
+    [Fact]
+    public void Router_only_commands_are_rejected_for_client_kind()
+    {
+        var matrix = new ClientCommandMatrix();
+        matrix.IsAllowed(ClientKind.Client, "RS+").ShouldBeFalse();
+        matrix.IsAllowed(ClientKind.Router, "RS+").ShouldBeTrue();
+    }
+}