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/Configuration/LoggingReloadTests.cs

@@ -0,0 +1,169 @@
+// Tests for ConfigReloader.ApplyLoggingChanges.
+// Go reference: golang/nats-server/server/reload.go — traceOption.Apply, debugOption.Apply.
+
+using NATS.Server;
+using NATS.Server.Configuration;
+using Shouldly;
+
+namespace NATS.Server.Core.Tests.Configuration;
+
+public class LoggingReloadTests
+{
+    // ─── helpers ────────────────────────────────────────────────────
+
+    private static NatsOptions BaseOpts() => new NatsOptions();
+
+    // ─── tests ──────────────────────────────────────────────────────
+
+    [Fact]
+    public void No_changes_returns_no_changes()
+    {
+        // Go reference: reload.go traceOption.Apply — no-op when flags unchanged
+        var old = BaseOpts();
+        var updated = BaseOpts();
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.HasChanges.ShouldBeFalse();
+        result.LevelChanged.ShouldBeFalse();
+        result.TraceChanged.ShouldBeFalse();
+        result.DebugChanged.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Level_changed_detected()
+    {
+        // Go reference: reload.go debugOption.Apply — enabling debug changes effective level
+        var old = BaseOpts();           // Debug=false, Trace=false → "Information"
+        var updated = BaseOpts();
+        updated.Debug = true;           // Debug=true → "Debug"
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.LevelChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Trace_enabled_detected()
+    {
+        // Go reference: reload.go traceOption.Apply — enabling trace flag
+        var old = BaseOpts();
+        var updated = BaseOpts();
+        updated.Trace = true;
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.TraceChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Trace_disabled_detected()
+    {
+        // Go reference: reload.go traceOption.Apply — disabling trace flag
+        var old = BaseOpts();
+        old.Trace = true;
+        var updated = BaseOpts();       // Trace=false
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.TraceChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Debug_enabled_detected()
+    {
+        // Go reference: reload.go debugOption.Apply — enabling debug flag
+        var old = BaseOpts();
+        var updated = BaseOpts();
+        updated.Debug = true;
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.DebugChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Debug_disabled_detected()
+    {
+        // Go reference: reload.go debugOption.Apply — disabling debug flag
+        var old = BaseOpts();
+        old.Debug = true;
+        var updated = BaseOpts();       // Debug=false
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.DebugChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void OldLevel_and_NewLevel_set()
+    {
+        // Go reference: reload.go — level transition is reported with explicit before/after values
+        var old = BaseOpts();           // Information
+        var updated = BaseOpts();
+        updated.Trace = true;           // Trace takes precedence
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.LevelChanged.ShouldBeTrue();
+        result.OldLevel.ShouldBe("Information");
+        result.NewLevel.ShouldBe("Trace");
+    }
+
+    [Fact]
+    public void Case_insensitive_level_comparison()
+    {
+        // Same effective level produced regardless of flag combination that yields the same tier
+        // Debug=true on both sides → "Debug" == "Debug", no level change
+        var old = BaseOpts();
+        old.Debug = true;
+        var updated = BaseOpts();
+        updated.Debug = true;
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.LevelChanged.ShouldBeFalse();
+        result.HasChanges.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Null_level_defaults_to_Information()
+    {
+        // Go reference: reload.go — absent log level is treated as Information
+        // When neither Debug nor Trace is set the effective level is "Information"
+        var old = BaseOpts();
+        var updated = BaseOpts();
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        // No change, and effective level should be "Information" for both sides
+        result.LevelChanged.ShouldBeFalse();
+        result.OldLevel.ShouldBeNull();  // only populated when a level change is detected
+        result.NewLevel.ShouldBeNull();
+    }
+
+    [Fact]
+    public void Multiple_changes_detected()
+    {
+        // Go reference: reload.go — independent trace and debug options both apply
+        var old = BaseOpts();
+        old.Debug = true;
+
+        var updated = BaseOpts();
+        updated.Trace = true;       // Debug removed, Trace added — both flags changed
+
+        var result = ConfigReloader.ApplyLoggingChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.TraceChanged.ShouldBeTrue();
+        result.DebugChanged.ShouldBeTrue();
+        result.LevelChanged.ShouldBeTrue();
+        result.OldLevel.ShouldBe("Debug");
+        result.NewLevel.ShouldBe("Trace");
+    }
+}