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/NatsStrictCapabilityInventoryTests.cs

@@ -0,0 +1,14 @@
+using NATS.Server.TestUtilities.Parity;
+
+namespace NATS.Server.Core.Tests.Parity;
+
+public class NatsStrictCapabilityInventoryTests
+{
+    [Fact]
+    public void Strict_capability_inventory_has_no_open_items_marked_done_without_behavior_and_tests()
+    {
+        var report = NatsCapabilityInventory.Load(
+            "docs/plans/2026-02-23-nats-strict-full-go-parity-map.md");
+        report.InvalidRows.ShouldBeEmpty();
+    }
+}