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/JetStreamConfigReloadTests.cs

@@ -0,0 +1,171 @@
+// Port of Go server/reload.go — JetStream config change detection tests.
+// Reference: golang/nats-server/server/reload.go — jetStreamOption.Apply.
+
+using NATS.Server.Configuration;
+using Shouldly;
+
+namespace NATS.Server.Core.Tests.Configuration;
+
+public sealed class JetStreamConfigReloadTests
+{
+    // ─── helpers ────────────────────────────────────────────────────
+
+    private static NatsOptions BaseOpts() => new();
+
+    private static NatsOptions OptsWithJs(long maxMemory = 0, long maxStore = 0, string? domain = null) =>
+        new()
+        {
+            JetStream = new JetStreamOptions
+            {
+                MaxMemoryStore = maxMemory,
+                MaxFileStore = maxStore,
+                Domain = domain
+            }
+        };
+
+    // ─── tests ──────────────────────────────────────────────────────
+
+    [Fact]
+    public void No_changes_returns_no_changes()
+    {
+        // Identical JetStream config on both sides → no changes detected.
+        var oldOpts = OptsWithJs(maxMemory: 512, maxStore: 1024, domain: "hub");
+        var newOpts = OptsWithJs(maxMemory: 512, maxStore: 1024, domain: "hub");
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.HasChanges.ShouldBeFalse();
+        result.MaxMemoryChanged.ShouldBeFalse();
+        result.MaxStoreChanged.ShouldBeFalse();
+        result.DomainChanged.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void MaxMemory_changed_detected()
+    {
+        // Changing MaxMemoryStore must set MaxMemoryChanged and HasChanges.
+        var oldOpts = OptsWithJs(maxMemory: 1024 * 1024);
+        var newOpts = OptsWithJs(maxMemory: 2 * 1024 * 1024);
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.MaxMemoryChanged.ShouldBeTrue();
+        result.HasChanges.ShouldBeTrue();
+        result.MaxStoreChanged.ShouldBeFalse();
+        result.DomainChanged.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void MaxStore_changed_detected()
+    {
+        // Changing MaxFileStore must set MaxStoreChanged and HasChanges.
+        var oldOpts = OptsWithJs(maxStore: 10 * 1024 * 1024);
+        var newOpts = OptsWithJs(maxStore: 20 * 1024 * 1024);
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.MaxStoreChanged.ShouldBeTrue();
+        result.HasChanges.ShouldBeTrue();
+        result.MaxMemoryChanged.ShouldBeFalse();
+        result.DomainChanged.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Domain_changed_detected()
+    {
+        // Changing the domain name must set DomainChanged and HasChanges.
+        var oldOpts = OptsWithJs(domain: "hub");
+        var newOpts = OptsWithJs(domain: "edge");
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.DomainChanged.ShouldBeTrue();
+        result.HasChanges.ShouldBeTrue();
+        result.MaxMemoryChanged.ShouldBeFalse();
+        result.MaxStoreChanged.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void OldMaxMemory_and_NewMaxMemory_set()
+    {
+        // When MaxMemoryStore changes the old and new values must be captured.
+        var oldOpts = OptsWithJs(maxMemory: 100_000);
+        var newOpts = OptsWithJs(maxMemory: 200_000);
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.OldMaxMemory.ShouldBe(100_000L);
+        result.NewMaxMemory.ShouldBe(200_000L);
+    }
+
+    [Fact]
+    public void OldMaxStore_and_NewMaxStore_set()
+    {
+        // When MaxFileStore changes the old and new values must be captured.
+        var oldOpts = OptsWithJs(maxStore: 500_000);
+        var newOpts = OptsWithJs(maxStore: 1_000_000);
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.OldMaxStore.ShouldBe(500_000L);
+        result.NewMaxStore.ShouldBe(1_000_000L);
+    }
+
+    [Fact]
+    public void OldDomain_and_NewDomain_set()
+    {
+        // When Domain changes both old and new values must be captured.
+        var oldOpts = OptsWithJs(domain: "alpha");
+        var newOpts = OptsWithJs(domain: "beta");
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.OldDomain.ShouldBe("alpha");
+        result.NewDomain.ShouldBe("beta");
+    }
+
+    [Fact]
+    public void Multiple_changes_detected()
+    {
+        // Changing MaxMemory, MaxStore, and Domain together must flag all three.
+        var oldOpts = OptsWithJs(maxMemory: 1024, maxStore: 2048, domain: "primary");
+        var newOpts = OptsWithJs(maxMemory: 4096, maxStore: 8192, domain: "secondary");
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.MaxMemoryChanged.ShouldBeTrue();
+        result.MaxStoreChanged.ShouldBeTrue();
+        result.DomainChanged.ShouldBeTrue();
+        result.HasChanges.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Zero_to_nonzero_memory_detected()
+    {
+        // Going from the default (0 = unlimited) to a concrete limit must be detected.
+        var oldOpts = BaseOpts(); // JetStream is null → effective MaxMemoryStore = 0
+        var newOpts = OptsWithJs(maxMemory: 512 * 1024 * 1024);
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.MaxMemoryChanged.ShouldBeTrue();
+        result.HasChanges.ShouldBeTrue();
+        result.OldMaxMemory.ShouldBe(0L);
+        result.NewMaxMemory.ShouldBe(512 * 1024 * 1024L);
+    }
+
+    [Fact]
+    public void Domain_null_to_value_detected()
+    {
+        // Going from no JetStream config (domain null/empty) to a named domain must be detected.
+        var oldOpts = BaseOpts(); // JetStream is null → effective domain = ""
+        var newOpts = OptsWithJs(domain: "cloud");
+
+        var result = ConfigReloader.ApplyJetStreamConfigChanges(oldOpts, newOpts);
+
+        result.DomainChanged.ShouldBeTrue();
+        result.HasChanges.ShouldBeTrue();
+        result.OldDomain.ShouldBe(string.Empty);
+        result.NewDomain.ShouldBe("cloud");
+    }
+}