Fix ToMarkdownTable to show raw values instead of record type names

C# record types auto-generate ToString() showing 'Text { IsNull = False, Value = 1 }'
which overrides the base SqlValue.ToString(). Use AsString() ?? "NULL" instead
so markdown output shows clean values like '1', 'true', 'NULL'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: vkuttyp

src/SqlDotnetty.Core/SqlDataTable.cs

@@ -38,9 +38,11 @@ public string ToMarkdownTable()
         var sb = new StringBuilder();
         var widths = Columns.Select((c, i) =>
             Math.Max(c.Name.Length, Rows.Count > 0
-                ? Rows.Max(r => r[i].ToString().Length)
+                ? Rows.Max(r => CellText(r[i]).Length)
                 : 0)).ToArray();
 
+        static string CellText(SqlValue v) => v.AsString() ?? "NULL";
+
         void Row(IEnumerable<string> cells)
         {
             sb.Append("| ");
@@ -54,7 +56,7 @@ void Row(IEnumerable<string> cells)
         sb.Append("| ");
         foreach (var w in widths) sb.Append(new string('-', w)).Append(" | ");
         sb.AppendLine();
-        foreach (var r in Rows) Row(Enumerable.Range(0, ColumnCount).Select(i => r[i].ToString()));
+        foreach (var r in Rows) Row(Enumerable.Range(0, ColumnCount).Select(i => CellText(r[i])));
         return sb.ToString();
     }
 }