feat(batch3): implement send queue feature group

dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Accounts.cs

@@ -334,6 +334,23 @@ public sealed partial class NatsServer
     public ClientConnection CreateInternalAccountClient() =>
         CreateInternalClient(ClientKind.Account);
 
+    /// <summary>
+    /// Creates and attaches a per-account send queue.
+    /// Mirrors Go <c>Server.newSendQ</c> call sites.
+    /// </summary>
+    internal SendQueue NewSendQueue(Account account)
+    {
+        ArgumentNullException.ThrowIfNull(account);
+
+        var existing = account.GetSendQueue();
+        if (existing is not null)
+            return existing;
+
+        var sendQueue = SendQueue.NewSendQ(this, account);
+        account.SetSendQueue(sendQueue);
+        return sendQueue;
+    }
+
     /// <summary>
     /// Creates an internal client of the given <paramref name="kind"/>.
     /// Mirrors Go <c>Server.createInternalClient</c>.