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/Protocol/MessageTraceInitializationTests.cs

@@ -0,0 +1,24 @@
+using NATS.Server.Protocol;
+
+namespace NATS.Server.Core.Tests;
+
+public class MessageTraceInitializationTests
+{
+    [Fact]
+    public void Trace_context_is_initialized_from_connect_options()
+    {
+        var connectOpts = new ClientOptions
+        {
+            Name = "c1",
+            Lang = "dotnet",
+            Version = "1.0.0",
+            Headers = true,
+        };
+
+        var ctx = MessageTraceContext.CreateFromConnect(connectOpts);
+        ctx.ClientName.ShouldBe("c1");
+        ctx.ClientLang.ShouldBe("dotnet");
+        ctx.ClientVersion.ShouldBe("1.0.0");
+        ctx.HeadersEnabled.ShouldBeTrue();
+    }
+}