fix(Payroll CTC Calculation): compute future non taxable earnings with full payment days

[ruchamahabal]

Before

Create a salary slip for a month with payment days < total working days

Employee joined on May 6th. Taxable Earnings from previous employer have been set in SSA (Note: this step is not mandatory to replicate the issue. Just a partial payment days salary slip should work):

May month salary:

Payment Days (26) < Total Working Days (31)

Income Tax Breakup

CTC calculation = gross pay

So here the calculation should have been:

Previous employer taxable earnings = 136577
May month salary (gross pay) = 1,44,747.04 (as per 26 payment days)
Upcoming 10-month salary = 172583 (full payment days salary) ×10 (remaining 10 months) = 1725830
Actual CTC = 2007154.04
System's calculated CTC = 2005863.74
Difference = 1290.3

Problem

This was how system was calculating future month's non taxable earnings:

future_period_non_taxable_earnings = current_period_non_taxable_earnings * (
	ceil(self.remaining_sub_periods) - 1
)

Here this reduces future earnings if current month has less payment days. Fixed it to consider full payment days for future tax calculations

After

• Check similar cases where future calculations use current calculations as anchor - (all other places already factor out payment days variable)
• Add tests