[18.0][REF] payroll: refactor start/end dates the month

[CristianoMafraJunior]

Root cause

The failing test was TestPayslipFlow.test_get_contracts_singleton in payroll/tests/test_payslip_flow.py.
This test was tweaking dates and creating a second contract for employee Sally using Date.today(), assuming that would always be safe.
In the current hr_contract implementation, there is a constraint (_check_current_contract) that forbids an employee from having two overlapping contracts in active-like states (incoming/open/close), except when they are draft or cancel.
Combined with the way Sally’s first contract is created in common.py (always starting on the first day of the month), this meant that around certain dates (especially month/year boundaries) the two contracts’ periods could overlap, triggering:
ValidationError: An employee can only have one contract at the same time. (Excluding Draft and Cancelled contracts).

Fix

Instead of relying on Date.today() inside the test, we now build the scenario based on the actual dates of Sally’s first contract, which is created in TestPayslipBase:
Retrieve Sally’s current contract: first_contract = self.sally.contract_id.
Close that contract with a non-overlapping, valid period:
date_end = first_contract.date_start
first_contract.write({"date_end": end_date.strftime("%Y-%m-%d"), "state": "close"})
Create the second contract starting the day after the first contract ends:
second_start = end_date + timedelta(days=1)
self.Contract.create({... "date_start": second_start.strftime("%Y-%m-%d"), ...})

With this:

There is no longer any overlap between Sally’s contracts.
The _check_current_contract constraint is satisfied.
The test still verifies the intended behavior — _get_employee_contracts() returns exactly one valid contract for the employee — but now in a deterministic, date‑independent way.

[OCA-git-bot]

Hi @nimarosa, @appstogrow,
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