This update restructures the project timesheet and employee leave integration to make it easier to maintain and extend. It addresses existing maintenance needs without introducing notable changes for day-to-day users.

Description of the issue/feature this PR addresses:
Addresses #30854 and regular code maintenance.

--
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

This update streamlines how project timesheets linked to time off handle date and time values. It removes an unnecessary conversion step, which helps keep the code cleaner and reduces the chance of date handling inconsistencies without changing user-facing behavior.

Description of the issue/feature this PR addresses:
No need to convert `datetime` to string

--
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr