fix(etl): address Codex MCP review findings for Phase 2

- Filter MERGE SQL columns to only include columns that exist in destination
  (allColumns and updateColumns were using unfiltered source columns)
- Fix schema-qualified table names to use proper [schema].[table] format
  instead of wrapping entire name in single brackets
- Add empty column mapping validation to throw early if no columns intersect
- Add JdeDateTransformer output column collision detection in OnInitialize
- Add TODO comment for WithCommandTimeout (stored but not yet passed to
  destinations)
- Add tests for FormatQualifiedTableName and output column collision

NEW/src/JdeScoping.DataSync/Etl/Scripts/CommonScripts.cs

@@ -18,6 +18,15 @@ public static class CommonScripts
             : ("dbo", parts[0]);
     }
 
+    /// <summary>
+    /// Formats a table name as a properly quoted [schema].[table] identifier.
+    /// </summary>
+    public static string FormatQualifiedTableName(string tableName)
+    {
+        var (schema, table) = ParseTableName(tableName);
+        return $"[{schema}].[{table}]";
+    }
+
     public static IScriptRunner DisableIndexes(
         IDbConnectionFactory factory,
         string tableName,