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/ClientKindTests.cs

@@ -0,0 +1,50 @@
+// Tests for ClientKind enum and IsInternal() extension method.
+// Go reference: client.go:45-65 (client kind constants and isInternal check)
+
+using NATS.Server;
+using Shouldly;
+
+namespace NATS.Server.Core.Tests;
+
+public class ClientKindTests
+{
+    [Fact]
+    public void Client_is_not_internal() =>
+        ClientKind.Client.IsInternal().ShouldBeFalse();
+
+    [Fact]
+    public void Router_is_not_internal() =>
+        ClientKind.Router.IsInternal().ShouldBeFalse();
+
+    [Fact]
+    public void Gateway_is_not_internal() =>
+        ClientKind.Gateway.IsInternal().ShouldBeFalse();
+
+    [Fact]
+    public void Leaf_is_not_internal() =>
+        ClientKind.Leaf.IsInternal().ShouldBeFalse();
+
+    [Fact]
+    public void System_is_internal() =>
+        ClientKind.System.IsInternal().ShouldBeTrue();
+
+    [Fact]
+    public void JetStream_is_internal() =>
+        ClientKind.JetStream.IsInternal().ShouldBeTrue();
+
+    [Fact]
+    public void Account_is_internal() =>
+        ClientKind.Account.IsInternal().ShouldBeTrue();
+
+    [Fact]
+    public void All_kinds_defined() =>
+        Enum.GetValues<ClientKind>().Length.ShouldBe(7);
+
+    [Fact]
+    public void Internal_kinds_count_is_three() =>
+        Enum.GetValues<ClientKind>().Count(k => k.IsInternal()).ShouldBe(3);
+
+    [Fact]
+    public void External_kinds_count_is_four() =>
+        Enum.GetValues<ClientKind>().Count(k => !k.IsInternal()).ShouldBe(4);
+}