test(batch18): port behavioral batch-18 tests

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/ImplBacklog/RouteHandlerTests.Impltests.cs

@@ -7,6 +7,22 @@ namespace ZB.MOM.NatsNet.Server.Tests.ImplBacklog;
 
 public sealed partial class RouteHandlerTests
 {
+    [Fact] // T:2819
+    public async Task RouteIPResolutionAndRouteToSelf_ShouldSucceed()
+    {
+        var (server, err) = NatsServer.NewServer(new ServerOptions());
+        err.ShouldBeNull();
+        server.ShouldNotBeNull();
+
+        var resolver = new FixedResolver(["127.0.0.1", "other.host.in.cluster"]);
+        var excluded = new HashSet<string>(StringComparer.Ordinal) { "127.0.0.1:1234" };
+
+        var (address, resolveErr) = await server!.GetRandomIP(resolver, "routehost:1234", excluded);
+
+        resolveErr.ShouldBeNull();
+        address.ShouldBe("other.host.in.cluster:1234");
+    }
+
     [Fact]
     public void NumRemotesInternal_WhenRoutesExist_ReturnsCount()
     {
@@ -122,4 +138,10 @@ public sealed partial class RouteHandlerTests
         route.FlushOutbound().ShouldBeTrue();
         route.Flags.IsSet(ClientFlags.IsSlowConsumer).ShouldBeFalse();
     }
+
+    private sealed class FixedResolver(string[] hosts) : INetResolver
+    {
+        public Task<string[]> LookupHostAsync(string host, CancellationToken ct = default)
+            => Task.FromResult(hosts);
+    }
 }