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/Internal/InternalDsPeriodicSamplerParityTests.cs

@@ -0,0 +1,37 @@
+using System.Reflection;
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Monitoring;
+
+namespace NATS.Server.Core.Tests.Internal;
+
+public class InternalDsPeriodicSamplerParityTests
+{
+    [Fact]
+    [SlopwatchSuppress("SW004", "Test must observe a real 1-second CPU sampling timer tick; wall-clock elapsed time is the observable under test")]
+    public async Task VarzHandler_uses_periodic_background_cpu_sampler()
+    {
+        var options = new NatsOptions { Host = "127.0.0.1", Port = 0 };
+        var server = new NatsServer(options, NullLoggerFactory.Instance);
+        using var cts = new CancellationTokenSource();
+        _ = server.StartAsync(cts.Token);
+        await server.WaitForReadyAsync();
+
+        try
+        {
+            using var handler = new VarzHandler(server, options, NullLoggerFactory.Instance);
+            var field = typeof(VarzHandler).GetField("_lastCpuSampleTime", BindingFlags.NonPublic | BindingFlags.Instance);
+            field.ShouldNotBeNull();
+
+            var before = (DateTime)field!.GetValue(handler)!;
+            await Task.Delay(TimeSpan.FromMilliseconds(1200));
+            var after = (DateTime)field.GetValue(handler)!;
+
+            after.ShouldBeGreaterThan(before);
+        }
+        finally
+        {
+            await cts.CancelAsync();
+            server.Dispose();
+        }
+    }
+}