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/Parity/JetStreamParityTruthMatrixTests.cs

@@ -0,0 +1,26 @@
+using NATS.Server.TestUtilities.Parity;
+
+namespace NATS.Server.Core.Tests.Parity;
+
+public class JetStreamParityTruthMatrixTests
+{
+    [Fact]
+    public void Jetstream_parity_rows_require_behavior_test_and_docs_alignment()
+    {
+        var report = JetStreamParityTruthMatrix.Load(
+            "differences.md",
+            "docs/plans/2026-02-23-jetstream-remaining-parity-map.md");
+
+        report.DriftRows.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void Jetstream_differences_notes_have_no_contradictions_against_status_table_and_truth_matrix()
+    {
+        var report = JetStreamParityTruthMatrix.Load(
+            "differences.md",
+            "docs/plans/2026-02-23-jetstream-remaining-parity-map.md");
+
+        report.Contradictions.ShouldBeEmpty();
+    }
+}