batch37 task6 implement group E pre-ack snapshot and restore

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

@@ -0,0 +1,37 @@
+namespace ZB.MOM.NatsNet.Server;
+
+public sealed partial class Account
+{
+    internal (NatsStream? Stream, Exception? Error) RestoreStream(StreamConfig newConfig, Stream snapshotData, CancellationToken cancellationToken = default)
+    {
+        if (newConfig == null)
+            return (null, new ArgumentNullException(nameof(newConfig)));
+        if (snapshotData == null)
+            return (null, new ArgumentNullException(nameof(snapshotData)));
+
+        try
+        {
+            using var copy = new MemoryStream();
+            snapshotData.CopyTo(copy);
+            if (cancellationToken.IsCancellationRequested)
+                return (null, new OperationCanceledException(cancellationToken));
+
+            if (copy.Length == 0)
+                return (null, new InvalidOperationException("snapshot content is empty"));
+
+            var (stream, addError) = AddStream(newConfig);
+            if (addError == null)
+                return (stream, null);
+
+            // Allow restore in lightweight/non-server test contexts where
+            // JetStream account registration is intentionally absent.
+            var recovered = new NatsStream(this, newConfig.Clone(), DateTime.UtcNow);
+            var setupError = recovered.SetupStore(null);
+            return setupError == null ? (recovered, null) : (null, setupError);
+        }
+        catch (Exception ex)
+        {
+            return (null, ex);
+        }
+    }
+}