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/ClientFlagsTests.cs

@@ -0,0 +1,53 @@
+namespace NATS.Server.Core.Tests;
+
+public class ClientFlagsTests
+{
+    [Fact]
+    public void SetFlag_and_HasFlag_work()
+    {
+        var holder = new ClientFlagHolder();
+        holder.HasFlag(ClientFlags.ConnectReceived).ShouldBeFalse();
+
+        holder.SetFlag(ClientFlags.ConnectReceived);
+        holder.HasFlag(ClientFlags.ConnectReceived).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void ClearFlag_removes_flag()
+    {
+        var holder = new ClientFlagHolder();
+        holder.SetFlag(ClientFlags.ConnectReceived);
+        holder.SetFlag(ClientFlags.IsSlowConsumer);
+
+        holder.ClearFlag(ClientFlags.ConnectReceived);
+
+        holder.HasFlag(ClientFlags.ConnectReceived).ShouldBeFalse();
+        holder.HasFlag(ClientFlags.IsSlowConsumer).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Multiple_flags_can_be_set_independently()
+    {
+        var holder = new ClientFlagHolder();
+        holder.SetFlag(ClientFlags.ConnectReceived);
+        holder.SetFlag(ClientFlags.WriteLoopStarted);
+        holder.SetFlag(ClientFlags.FirstPongSent);
+
+        holder.HasFlag(ClientFlags.ConnectReceived).ShouldBeTrue();
+        holder.HasFlag(ClientFlags.WriteLoopStarted).ShouldBeTrue();
+        holder.HasFlag(ClientFlags.FirstPongSent).ShouldBeTrue();
+        holder.HasFlag(ClientFlags.IsSlowConsumer).ShouldBeFalse();
+    }
+
+    [Fact]
+    public void SetFlag_is_thread_safe()
+    {
+        var holder = new ClientFlagHolder();
+        var flags = Enum.GetValues<ClientFlags>();
+
+        Parallel.ForEach(flags, flag => holder.SetFlag(flag));
+
+        foreach (var flag in flags)
+            holder.HasFlag(flag).ShouldBeTrue();
+    }
+}