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/ClientClosedReasonTests.cs

@@ -0,0 +1,24 @@
+namespace NATS.Server.Core.Tests;
+
+public class ClientClosedReasonTests
+{
+    [Fact]
+    public void All_expected_close_reasons_exist()
+    {
+        // Verify all 18 enum values exist and are distinct (None + 17 named reasons)
+        var values = Enum.GetValues<ClientClosedReason>();
+        values.Length.ShouldBe(18);
+        values.Distinct().Count().ShouldBe(18);
+    }
+
+    [Theory]
+    [InlineData(ClientClosedReason.ClientClosed, "Client Closed")]
+    [InlineData(ClientClosedReason.SlowConsumerPendingBytes, "Slow Consumer (Pending Bytes)")]
+    [InlineData(ClientClosedReason.SlowConsumerWriteDeadline, "Slow Consumer (Write Deadline)")]
+    [InlineData(ClientClosedReason.StaleConnection, "Stale Connection")]
+    [InlineData(ClientClosedReason.ServerShutdown, "Server Shutdown")]
+    public void ToReasonString_returns_human_readable_description(ClientClosedReason reason, string expected)
+    {
+        reason.ToReasonString().ShouldBe(expected);
+    }
+}