This commit fixes an issue in the Salary Calculator where all fields were displayed as empty when selecting an employee with a resource_calendar_id set.

Steps to Reproduce :
	First Change to either the default company (MyCompany) or Demo Belgian company
	Payroll -> Employee -> Salary Calculator -> choose an employee.

Bug : All fields in the Salary Calculator view are empty (evaluated as False).

Root Cause :

Inside _compute_salary, the method _generate_salary_simulation_payslip writes on payslip.version_id using:

`payslip.version_id.write(new_payslip_vals)`

Because the is_simulation_offer key was missing from the context, this write triggered a full payslip computation, generating payslip lines. During this computation, compute_sheet() performs an unlink() on existing payslip lines. Since the Salary Calculator view fields are only cached at that point, the unlink causes the cached values to be lost, resulting in all fields being evaluated as False.

Fix : Ensure that is_simulation_offer is present in the context when writing to payslip.version_id, preventing payslip line generation and avoiding the unintended unlink() during salary simulation.

Task - 5387155