batch36 task4 stream mirror lifecycle helpers

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/StreamLifecycleGroupBTests.cs

@@ -123,6 +123,44 @@ public sealed class StreamLifecycleGroupBTests
         jsa.SubjectsOverlap(["orders.created"], ownAssignment: null).ShouldBeTrue();
     }
 
+    [Fact]
+    public void GroupCHelpers_GetCfgNameAndIsMirror_ReturnExpectedValues()
+    {
+        var mirrorCfg = new StreamConfig
+        {
+            Name = "MIRROR",
+            Storage = StorageType.MemoryStorage,
+            Mirror = new StreamSource { Name = "ORIGIN" },
+        };
+
+        var stream = CreateStream(mirrorCfg);
+
+        stream.GetCfgName().ShouldBe("MIRROR");
+        stream.IsMirror().ShouldBeTrue();
+        stream.MirrorInfo().ShouldBeNull();
+    }
+
+    [Fact]
+    public void GroupCHelpers_SourceInfoAndBackoff_Behave()
+    {
+        var stream = CreateStream();
+        var info = new StreamSourceInfo
+        {
+            Name = "SRC",
+            FilterSubject = "orders.*",
+            Lag = 10,
+            Error = "x",
+        };
+
+        var cloned = stream.SourceInfo(info);
+        cloned.ShouldNotBeNull();
+        cloned!.Name.ShouldBe("SRC");
+        cloned.FilterSubject.ShouldBe("orders.*");
+
+        NatsStream.CalculateRetryBackoff(1).ShouldBeGreaterThan(TimeSpan.Zero);
+        NatsStream.CalculateRetryBackoff(1000).ShouldBe(TimeSpan.FromMinutes(2));
+    }
+
     private static NatsStream CreateStream(StreamConfig? cfg = null)
     {
         cfg ??= new StreamConfig { Name = "ORDERS", Subjects = ["orders.*"], Storage = StorageType.MemoryStorage };