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/LoggingTests.cs

@@ -0,0 +1,60 @@
+using Serilog;
+
+namespace NATS.Server.Core.Tests;
+
+public class LoggingTests : IDisposable
+{
+    private readonly string _logDir;
+
+    public LoggingTests()
+    {
+        _logDir = Path.Combine(Path.GetTempPath(), $"nats-log-test-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(_logDir);
+    }
+
+    public void Dispose()
+    {
+        try { Directory.Delete(_logDir, true); } catch { }
+    }
+
+    [Fact]
+    public void File_sink_creates_log_file()
+    {
+        var logPath = Path.Combine(_logDir, "test.log");
+
+        using var logger = new LoggerConfiguration()
+            .WriteTo.File(logPath)
+            .CreateLogger();
+
+        logger.Information("Hello from test");
+        logger.Dispose();
+
+        File.Exists(logPath).ShouldBeTrue();
+        var content = File.ReadAllText(logPath);
+        content.ShouldContain("Hello from test");
+    }
+
+    [Fact]
+    public void File_sink_rotates_on_size_limit()
+    {
+        var logPath = Path.Combine(_logDir, "rotate.log");
+
+        using var logger = new LoggerConfiguration()
+            .WriteTo.File(
+                logPath,
+                fileSizeLimitBytes: 200,
+                rollOnFileSizeLimit: true,
+                retainedFileCountLimit: 3)
+            .CreateLogger();
+
+        // Write enough to trigger rotation
+        for (int i = 0; i < 50; i++)
+            logger.Information("Log message number {Number} with some padding text", i);
+
+        logger.Dispose();
+
+        // Should have created rotated files
+        var logFiles = Directory.GetFiles(_logDir, "rotate*.log");
+        logFiles.Length.ShouldBeGreaterThan(1);
+    }
+}