fix(#2086): correct payroll tax withholding undercalculation on bonuses

[Vachhani-Tapan]

Fix: Payroll Tax Withholding Undercalculation on Bonuses

Resolves#2086

Description

This PR addresses the critical issue where one-time bonus payments were being under-taxed by placing employees in lower marginal tax brackets instead of applying supplemental or annualized tax rates.

Bonus payments are now appropriately evaluated distinctly from regular monthly income, allowing for much more accurate statutory withholding for high-earning individuals.

Code Changes

• Added taxOnBonus Utility: Implemented a new calculation strategy inside backend/src/utils/taxCalculator.js capable of processing bonus taxation using either a SUPPLEMENTAL flat-rate method (defaulting to 22%) or the AGGREGATE method (which marginalizes the bonus over the annualized base salary).
• Integrated into TaxService: Updated calculateTax() in backend/src/services/tax.service.js to accept bonusIncome and an optional bonusTaxMethod. The service now securely separates and aggregates base and bonus taxation logic without disrupting old legacy routines.
• Added Unit Tests: Included comprehensive testing in taxCalculator.test.js to ensure the AGGREGATE marginal difference operates correctly on large bonuses and falls back to flat-rate supplemental rules securely.

Verification

• Passed all slab and progressive calculation unit tests (node --test).
• No breaking changes to existing regular payroll configurations or deductions.

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