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/SignalHandlerTests.cs

@@ -0,0 +1,61 @@
+using NATS.Server.Configuration;
+
+namespace NATS.Server.Core.Tests;
+
+// Go reference: server/signal_unix.go (handleSignals), server/reload.go (Reload)
+
+public class SignalHandlerTests
+{
+    [Fact]
+    public void SignalHandler_registers_without_throwing()
+    {
+        // Go reference: server/signal_unix.go — registration should succeed
+        Should.NotThrow(() => SignalHandler.Register(() => { }));
+        SignalHandler.IsRegistered.ShouldBeTrue();
+        SignalHandler.Unregister();
+        SignalHandler.IsRegistered.ShouldBeFalse();
+    }
+
+    [Fact]
+    public async Task ConfigReloader_ReloadFromOptionsAsync_applies_reloadable_changes()
+    {
+        // Go reference: server/reload.go — reloadable options (e.g., MaxPayload) pass validation
+        var original = new NatsOptions { Port = 4222, MaxPayload = 1024 };
+        var updated = new NatsOptions { Port = 4222, MaxPayload = 2048 }; // MaxPayload is reloadable
+
+        var result = await ConfigReloader.ReloadFromOptionsAsync(original, updated);
+        result.Success.ShouldBeTrue();
+        result.RejectedChanges.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public async Task ConfigReloader_rejects_non_reloadable_changes()
+    {
+        // Go reference: server/reload.go — Port change is NOT reloadable
+        var original = new NatsOptions { Port = 4222 };
+        var updated = new NatsOptions { Port = 5555 }; // port change is NOT reloadable
+
+        var result = await ConfigReloader.ReloadFromOptionsAsync(original, updated);
+        result.RejectedChanges.ShouldContain(c => c.Contains("Port"));
+    }
+
+    [Fact]
+    public async Task ConfigReloader_identical_options_succeeds()
+    {
+        // Go reference: server/reload.go — no changes = success
+        var original = new NatsOptions { Port = 4222 };
+        var updated = new NatsOptions { Port = 4222 };
+
+        var result = await ConfigReloader.ReloadFromOptionsAsync(original, updated);
+        result.Success.ShouldBeTrue();
+        result.RejectedChanges.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void SignalHandler_unregister_is_idempotent()
+    {
+        // Calling Unregister when not registered should not throw
+        Should.NotThrow(() => SignalHandler.Unregister());
+        Should.NotThrow(() => SignalHandler.Unregister());
+    }
+}