Fix: Round amount_finaid() to nearest cent to unblock percentage-aid payments - #6084

Percentage-based financial aid grants on non-whole-dollar amounts (e.g. 25% of $47.50 = $11.875) produce fractional cents in amount_finaid(). This makes amount_due() a non-integer number of cents, causing the Stripe payment flow to fail with an inconsistent_amount error because int(amount_due * 100) disagrees with Decimal(amount_due) * 100.

Round amount_finaid() using ROUND_HALF_UP per maintainer guidance so that all downstream accounting calculations resolve to whole cents. Add regression tests covering fractional cents and half-cent rounding.

Description

When financial aid is awarded as a percentage discount (e.g., 25%) and the program cost or item cost is not a whole-dollar amount (e.g., $47.50), IndividualAccountingController.amount_finaid() calculated fractional-cent aid amounts without rounding ($11.875).

This produced a cascading failure when the student attempted to pay online:

1. amount_due() returned fractional cents ($35.625).
2. payonline() computed the Stripe form amount as int(Decimal('35.625') * 100) = 3562 ($35.62).
3. charge_payment() verified this against Decimal(iac.amount_due()) * 100 = 3562.5.
4. Because 3562 ≠ 3562.5, charge_payment() aborted with inconsistent_amount, completely blocking the student from completing payment.
5. In addition, link_paid_transfers() would fail due to the transfers not summing to the target price ($35.62 + $11.875 = $47.495 ≠ $47.50).

Solution

• Updated IndividualAccountingController.amount_finaid() in esp/esp/accounting/controllers.py to round the final aid amount to whole cents using .quantize(Decimal('0.01'), rounding=ROUND_HALF_UP).
• Followed maintainer guidance on rounding mode (ROUND_HALF_UP: half a cent or more rounds up to the student's benefit; less than half a cent rounds down).
• Added 3 regression tests in esp/esp/accounting/tests/test_models.py verifying:
  • Exact issue scenario (25% off $47.50) produces whole cents and consistent payment amounts.
  • Half-cent boundary explicitly rounds up (confirming ROUND_HALF_UP).
  • Combination of dollar-cap grant and percentage discount resolves to whole cents.

Related Issue

Closes #6055

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactor / cleanup
• CI/CD or build change

Testing

• Existing tests pass
• New tests added (if applicable)
• Manually verified

Testing Details:

• New tests added:
  • IndividualAccountingControllerTest.test_amount_finaid_percentage_rounded_to_cents
  • IndividualAccountingControllerTest.test_amount_finaid_half_cent_rounds_up
  • IndividualAccountingControllerTest.test_amount_finaid_percentage_and_dollar_cap_rounded_to_cents
• Manual Verification:
  • Verified math on test cases ($47.50 at 25% → finaid $11.88, due $35.62; int(due * 100) == int(Decimal(due) * 100) == 3562).

AI Disclosure

This pull request was prepared with the assistance of an AI assistant (Google Antigravity) to identify root cause code paths, implement the rounding fix following maintainer discussion on #6055, and write regression tests. All changes and tests have been reviewed and verified.

Checklist

• Self-reviewed the code
• Updated documentation (if needed)
• No new warnings or errors introduced

Summary by CodeRabbit

• Bug Fixes
  • Financial aid amounts are now rounded to the nearest cent, keeping aid and remaining balances accurate in calculations. This applies to percentage-based grants and combinations of fixed-amount caps with percentage-based aid, helping ensure the amount due is consistently calculated and displayed in cents.