feat: complete legacy UI and playout parity migration

tests/MBN_STOCK_WEBVIEW.DbWriteSmoke.Tests/ExistingChildRowGuardTests.cs

@@ -0,0 +1,77 @@
+#nullable enable
+
+using System.Data;
+using MBN_STOCK_WEBVIEW.DbWriteSmoke;
+using MMoneyCoderSharp.Data;
+
+namespace MBN_STOCK_WEBVIEW.DbWriteSmoke.Tests;
+
+public sealed class ExistingChildRowGuardTests
+{
+    [Fact]
+    public async Task ZeroCount_UsesBoundProductionSourceSpecificPreflights()
+    {
+        var executor = new RecordingExecutor("0");
+        var guard = new ExistingChildRowGuard(executor);
+
+        await guard.EnsureNamedPlaylistChildrenAbsentAsync("00000042", default);
+        await guard.EnsureThemeChildrenAbsentAsync(ThemeMarket.Krx, "00000043", default);
+        await guard.EnsureThemeChildrenAbsentAsync(ThemeMarket.Nxt, "00000044", default);
+        await guard.EnsureExpertChildrenAbsentAsync("0045", default);
+
+        Assert.Equal(4, executor.Calls.Count);
+        Assert.Equal(
+            [DataSourceKind.Oracle, DataSourceKind.Oracle, DataSourceKind.MariaDb, DataSourceKind.Oracle],
+            executor.Calls.Select(call => call.Source));
+        Assert.All(executor.Calls, call =>
+        {
+            var parameter = Assert.Single(call.Spec.Parameters);
+            Assert.Equal("parent_code", parameter.Name);
+            Assert.Equal(DbType.String, parameter.DbType);
+            Assert.DoesNotContain((string)parameter.Value!, call.Spec.Sql, StringComparison.Ordinal);
+        });
+    }
+
+    [Fact]
+    public async Task ExistingOrInvalidChildCount_FailsClosedBeforeMutation()
+    {
+        var guard = new ExistingChildRowGuard(new RecordingExecutor("1"));
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            guard.EnsureNamedPlaylistChildrenAbsentAsync("00000042", default));
+    }
+
+    private sealed class RecordingExecutor(string count) : IDataQueryExecutor
+    {
+        public List<QueryCall> Calls { get; } = [];
+
+        public DataTable Execute(DataSourceKind source, string tableName, string query) =>
+            throw new NotSupportedException();
+
+        public Task<DataTable> ExecuteAsync(
+            DataSourceKind source,
+            string tableName,
+            string query,
+            CancellationToken cancellationToken = default) =>
+            throw new NotSupportedException();
+
+        public Task<DataTable> ExecuteAsync(
+            DataSourceKind source,
+            string tableName,
+            DataQuerySpec query,
+            CancellationToken cancellationToken = default)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            Calls.Add(new QueryCall(source, tableName, query));
+            var table = new DataTable();
+            table.Columns.Add("ROW_COUNT", typeof(string));
+            table.Rows.Add(count);
+            return Task.FromResult(table);
+        }
+    }
+
+    private sealed record QueryCall(
+        DataSourceKind Source,
+        string QueryName,
+        DataQuerySpec Spec);
+}