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/SlopwatchSuppressAttribute.cs

@@ -0,0 +1,12 @@
+// Marker attribute recognised by the slopwatch static-analysis tool.
+// Apply to a test method to suppress a specific slopwatch rule violation.
+// The justification must be 20+ characters explaining why the suppression is intentional.
+
+namespace NATS.Server.Core.Tests;
+
+[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
+public sealed class SlopwatchSuppressAttribute(string ruleId, string justification) : Attribute
+{
+    public string RuleId { get; } = ruleId;
+    public string Justification { get; } = justification;
+}