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/ResponseTrackerTests.cs

@@ -0,0 +1,51 @@
+using NATS.Server.Auth;
+
+namespace NATS.Server.Core.Tests;
+
+public class ResponseTrackerTests
+{
+    [Fact]
+    public void Allows_reply_subject_after_registration()
+    {
+        var tracker = new ResponseTracker(maxMsgs: 1, expires: TimeSpan.FromMinutes(5));
+        tracker.RegisterReply("_INBOX.abc123");
+        tracker.IsReplyAllowed("_INBOX.abc123").ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Denies_unknown_reply_subject()
+    {
+        var tracker = new ResponseTracker(maxMsgs: 1, expires: TimeSpan.FromMinutes(5));
+        tracker.IsReplyAllowed("_INBOX.unknown").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Enforces_max_messages()
+    {
+        var tracker = new ResponseTracker(maxMsgs: 2, expires: TimeSpan.FromMinutes(5));
+        tracker.RegisterReply("_INBOX.abc");
+        tracker.IsReplyAllowed("_INBOX.abc").ShouldBeTrue();
+        tracker.IsReplyAllowed("_INBOX.abc").ShouldBeTrue();
+        tracker.IsReplyAllowed("_INBOX.abc").ShouldBeFalse(); // exceeded
+    }
+
+    [Fact]
+    public void Enforces_expiry()
+    {
+        var tracker = new ResponseTracker(maxMsgs: 0, expires: TimeSpan.FromMilliseconds(1));
+        tracker.RegisterReply("_INBOX.abc");
+        Thread.Sleep(50);
+        tracker.IsReplyAllowed("_INBOX.abc").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Prune_removes_expired()
+    {
+        var tracker = new ResponseTracker(maxMsgs: 0, expires: TimeSpan.FromMilliseconds(1));
+        tracker.RegisterReply("_INBOX.a");
+        tracker.RegisterReply("_INBOX.b");
+        Thread.Sleep(50);
+        tracker.Prune();
+        tracker.Count.ShouldBe(0);
+    }
+}