client/dotnet: make LazyBrowseNode.ExpandAsync thread-safe

clients/dotnet/ZB.MOM.WW.MxGateway.Client.Tests/LazyBrowseNodeTests.cs

@@ -142,6 +142,37 @@ public sealed class LazyBrowseNodeTests
         Assert.Equal("7:abc:2", transport.BrowseChildrenCalls[2].Request.PageToken);
     }
 
+    /// <summary>
+    /// Verifies that ten concurrent ExpandAsync calls issue exactly one RPC, not ten.
+    /// </summary>
+    [Fact]
+    public async Task Expand_CalledConcurrently_OnlyFiresOneRpc()
+    {
+        FakeGalaxyRepositoryTransport transport = CreateTransport();
+        transport.BrowseChildrenReplies.Enqueue(BuildReply(
+            children: [BuildObject(1, "Plant", isArea: true)],
+            childHasChildren: [true],
+            cacheSequence: 7));
+        transport.BrowseChildrenReplies.Enqueue(BuildReply(
+            children: [BuildObject(2, "Mixer_001")],
+            childHasChildren: [false],
+            cacheSequence: 7));
+
+        await using GalaxyRepositoryClient client = CreateClient(transport);
+        IReadOnlyList<LazyBrowseNode> roots = await client.BrowseAsync();
+
+        // Fire ten concurrent expands of the same node.
+        Task[] tasks = Enumerable.Range(0, 10)
+            .Select(_ => roots[0].ExpandAsync())
+            .ToArray();
+        await Task.WhenAll(tasks);
+
+        Assert.True(roots[0].IsExpanded);
+        Assert.Single(roots[0].Children);
+        // 1 roots fetch + exactly 1 expand fetch = 2 total
+        Assert.Equal(2, transport.BrowseChildrenCalls.Count);
+    }
+
     /// <summary>
     /// Verifies that BrowseChildrenOptions filter fields are forwarded to the BrowseChildren request.
     /// </summary>