Original PR description

In [^1], during a small refactor of the `sale.order.is_late` method to only account for logistic delays rather than rental delays, the rental extra delay margin computation was moved to the
`sale.order.line._generate_delay_line` method. However, the computation used the utility class `relativedelta` instead of `timedelta`, which transformed the final rental duration into a `relativedelta` object.

This object internally stores second durations differently than a `timedelta`. While `timedelta` stores durations as `days` and remaining `seconds`, `relativedelta` splits the information into `days`, `hours`, `minutes`, etc. Since rental orders only account for hours, the `seconds` field was null, resulting in no extra delay fee being added to the order upon return.

This commit fixes the issue by using `timedelta` as expected.

[^1]: https://github.com/odoo/enterprise/pull/88689