feat: complete legacy parity and modernize operator UI

tests/MBN_STOCK_WEBVIEW.LegacyApplication.Tests/Expert/LegacyExpertWorkflowTests.cs

@@ -110,6 +110,38 @@ public sealed class LegacyExpertWorkflowTests
         Assert.Equal("1/0", draft.PageText);
     }
 
+    [Fact]
+    public async Task NullBuyAmount_RemainsVisibleAndCanBeComposedForFreshPrepareValidation()
+    {
+        var service = new DelegateExpertService
+        {
+            Search = (query, _, _) => Task.FromResult(SearchResult(query, Alpha)),
+            Preview = (identity, _, _) => Task.FromResult(PreviewResult(identity,
+            [
+                new ExpertRecommendationPreview(0, "005930", "Samsung", null),
+                new ExpertRecommendationPreview(1, "035720", "Kakao", 42_000m)
+            ]))
+        };
+        var controller = new LegacyExpertWorkflowController(service);
+        var search = await controller.SearchAsync(string.Empty);
+
+        var selected = await controller.SelectExpertAsync(search.Search.Results[0].SelectionId);
+
+        Assert.Equal(LegacyExpertRequestStatus.Ready, selected.Preview.Status);
+        Assert.Equal(2, selected.Preview.Items.Count);
+        Assert.Null(selected.Preview.Items[0].BuyAmount);
+        Assert.True(Assert.Single(selected.Actions).IsAvailable);
+
+        var draft = controller.MaterializeSelectedAction(
+            LegacyExpertWorkflowController.ActionId);
+
+        Assert.Equal(2, draft.ItemCount);
+        Assert.Equal(1, draft.PageCount);
+        Assert.Equal("1/1", draft.PageText);
+        Assert.Equal("0001", draft.ExpertCode);
+        Assert.Equal(Alpha.ExpertName, draft.ExpertName);
+    }
+
     [Fact]
     public async Task LateSearchResponse_CannotReplaceNewerExpertResult()
     {