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/NatsOptionsTests.cs

@@ -0,0 +1,54 @@
+namespace NATS.Server.Core.Tests;
+
+public class NatsOptionsTests
+{
+    [Fact]
+    public void Defaults_are_correct()
+    {
+        var opts = new NatsOptions();
+        opts.MaxSubs.ShouldBe(0);
+        opts.MaxSubTokens.ShouldBe(0);
+        opts.Debug.ShouldBe(false);
+        opts.Trace.ShouldBe(false);
+        opts.LogFile.ShouldBeNull();
+        opts.LogSizeLimit.ShouldBe(0L);
+        opts.Tags.ShouldBeNull();
+    }
+
+    [Fact]
+    public void New_fields_have_correct_defaults()
+    {
+        var opts = new NatsOptions();
+        opts.ClientAdvertise.ShouldBeNull();
+        opts.TraceVerbose.ShouldBeFalse();
+        opts.MaxTracedMsgLen.ShouldBe(0);
+        opts.DisableSublistCache.ShouldBeFalse();
+        opts.ConnectErrorReports.ShouldBe(3600);
+        opts.ReconnectErrorReports.ShouldBe(1);
+        opts.NoHeaderSupport.ShouldBeFalse();
+        opts.MaxClosedClients.ShouldBe(10_000);
+        opts.NoSystemAccount.ShouldBeFalse();
+        opts.SystemAccount.ShouldBeNull();
+    }
+}
+
+public class LogOverrideTests
+{
+    [Fact]
+    public void LogOverrides_defaults_to_null()
+    {
+        var options = new NatsOptions();
+        options.LogOverrides.ShouldBeNull();
+    }
+
+    [Fact]
+    public void LogOverrides_can_be_set()
+    {
+        var options = new NatsOptions
+        {
+            LogOverrides = new() { ["NATS.Server.Protocol"] = "Trace" }
+        };
+        options.LogOverrides.ShouldNotBeNull();
+        options.LogOverrides["NATS.Server.Protocol"].ShouldBe("Trace");
+    }
+}