refactor(mdb): use safe conn.execute() for fetch_table_row_count

[lewiszlw]

Summary

Replace preallocate + unsafe exec_direct + unsafe bind_col + unsafe fetch (3 unsafe blocks) in fetch_table_row_count with odbc-api's safe conn.execute() + cursor.next_row().

Before

let pre = conn.preallocate()?;
let mut stmt = pre.into_handle();
unsafe { stmt.exec_direct(&sql_text) }?;
unsafe { stmt.bind_col(1, &mut dummy) }?;
loop { match unsafe { stmt.fetch() } { ... } }

After

let mut cursor = conn.execute(&count_query, (), None)?...;
loop { match cursor.next_row() { ... } }

Zero unsafe blocks.