feat: execute post-baseline jetstream parity plan

src/NATS.Server/Gateways/ReplyMapper.cs

@@ -0,0 +1,29 @@
+namespace NATS.Server.Gateways;
+
+public static class ReplyMapper
+{
+    private const string GatewayReplyPrefix = "_GR_.";
+
+    public static string? ToGatewayReply(string? replyTo, string localClusterId)
+    {
+        if (string.IsNullOrWhiteSpace(replyTo))
+            return replyTo;
+
+        return $"{GatewayReplyPrefix}{localClusterId}.{replyTo}";
+    }
+
+    public static bool TryRestoreGatewayReply(string? gatewayReply, out string restoredReply)
+    {
+        restoredReply = string.Empty;
+
+        if (string.IsNullOrWhiteSpace(gatewayReply) || !gatewayReply.StartsWith(GatewayReplyPrefix, StringComparison.Ordinal))
+            return false;
+
+        var clusterSeparator = gatewayReply.IndexOf('.', GatewayReplyPrefix.Length);
+        if (clusterSeparator < 0 || clusterSeparator == gatewayReply.Length - 1)
+            return false;
+
+        restoredReply = gatewayReply[(clusterSeparator + 1)..];
+        return true;
+    }
+}