`query_first_row` function breaks for queries that end in a comment

When copying data with `DbToDbOperator` using a query, if the query provided by the user ends in a comment line, the `query_first_row` function will break with a syntax error, because the closing parenthesis will be inserted inside the comment.

**Proposed solution:** add a newline before and after the original query, when wrapping the original query as a subquery.