refactor: extract NATS.Server.Gateways.Tests project

Move 25 gateway-related test files from NATS.Server.Tests into a
dedicated NATS.Server.Gateways.Tests project. Update namespaces,
replace private ReadUntilAsync with SocketTestHelper from TestUtilities,
inline TestServerFactory usage, add InternalsVisibleTo, and register
the project in the solution file. All 261 tests pass.

tests/NATS.Server.Gateways.Tests/Gateways/GatewayInterestOnlyParityTests.cs

@@ -0,0 +1,17 @@
+using NATS.Server.Gateways;
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Gateways.Tests;
+
+public class GatewayInterestOnlyParityTests
+{
+    [Fact]
+    public void Gateway_interest_only_mode_forwards_only_subjects_with_remote_interest()
+    {
+        using var subList = new SubList();
+        subList.ApplyRemoteSub(new RemoteSubscription("orders.*", null, "gw1", "A"));
+
+        GatewayManager.ShouldForwardInterestOnly(subList, "A", "orders.created").ShouldBeTrue();
+        GatewayManager.ShouldForwardInterestOnly(subList, "A", "payments.created").ShouldBeFalse();
+    }
+}