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/GatewayAdvancedSemanticsTests.cs

@@ -0,0 +1,20 @@
+using NATS.Server.Gateways;
+
+namespace NATS.Server.Gateways.Tests;
+
+public class GatewayAdvancedSemanticsTests
+{
+    [Fact]
+    public void Gateway_forwarding_remaps_reply_subject_with_gr_prefix_and_restores_on_return()
+    {
+        const string originalReply = "_INBOX.123";
+        const string clusterId = "CLUSTER-A";
+
+        var mapped = ReplyMapper.ToGatewayReply(originalReply, clusterId);
+        mapped.ShouldStartWith("_GR_.");
+        mapped.ShouldContain(clusterId);
+
+        ReplyMapper.TryRestoreGatewayReply(mapped, out var restored).ShouldBeTrue();
+        restored.ShouldBe(originalReply);
+    }
+}