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/GatewayAdvancedRemapRuntimeTests.cs

@@ -0,0 +1,19 @@
+using NATS.Server.Gateways;
+
+namespace NATS.Server.Gateways.Tests;
+
+public class GatewayAdvancedRemapRuntimeTests
+{
+    [Fact]
+    public void Transport_internal_reply_and_loop_markers_never_leak_to_client_visible_subjects()
+    {
+        const string clientReply = "_INBOX.123";
+        var nested = ReplyMapper.ToGatewayReply(
+            ReplyMapper.ToGatewayReply(clientReply, "CLUSTER-A"),
+            "CLUSTER-B");
+
+        ReplyMapper.TryRestoreGatewayReply(nested, out var restored).ShouldBeTrue();
+        restored.ShouldBe(clientReply);
+        restored.ShouldNotStartWith("_GR_.");
+    }
+}