Comparison fields validation of large decimal fails on PostgreSQL

You can enable the column `col_num_38` in test `test_row_validation_comp_fields_oracle_to_postgres` to reproduce the problem.

In column validation we cast column that will overflow int64/float64 to string in the engine before we let Ibis/Pandas coerce them to float thereby making them lossy. I suggest we do the same for comparison fields.

This issue was found during internal testing and is not affecting a customer.