feat(batch27): implement jetstream config validation and error tail

dotnet/src/ZB.MOM.NatsNet.Server/Accounts/Account.JetStream.cs

@@ -111,6 +111,22 @@ public sealed partial class Account
         return EnableAllJetStreamServiceImportsAndMappings();
     }
 
+    internal (NatsServer? Server, JsAccount? JetStreamAccount, Exception? Error) CheckForJetStream()
+    {
+        _mu.EnterReadLock();
+        var server = Server as NatsServer;
+        var jsa = JetStream;
+        _mu.ExitReadLock();
+
+        if (server == null || jsa == null)
+        {
+            var description = JsApiErrors.NewJSNotEnabledForAccountError().Description ?? "jetstream not enabled for account";
+            return (null, null, new InvalidOperationException(description));
+        }
+
+        return (server, jsa, null);
+    }
+
     internal (bool MaxBytesRequired, long MaxStreamBytes) MaxBytesLimits(StreamConfig? cfg)
     {
         _mu.EnterReadLock();