Add car loan estimator: domain, service, API, validation, and tests

[jmpompeo]

Motivation

• Provide a car loan estimation feature that computes amount financed, monthly payment, total paid, total interest and an amortization schedule while supporting sales tax as either percent or amount.
• Expose the car loan calculation through the existing API surface with request validation and consistent response formatting.

Description

• Added domain request and response types: CarLoanRequest and CarLoanResult (and CarLoanAmortizationEntry) with input validation and rounding/display formatting logic.
• Implemented car loan calculation in CalculationService via CalculateCarLoanEstimate and a BuildAmortizationSchedule helper that produces a month-by-month amortization list.
• Updated the service contract ICalculationService to include CalculateCarLoanEstimate.
• Added API surface: CarLoanController, request DTO CarLoanEstimateRequestDto, response DTO CarLoanEstimateResponseDto, and mapping logic in CalculationMapper.
• Added request validation with CarLoanEstimateRequestValidator (FluentValidation) ensuring value ranges and that exactly one of salesTaxPercent or salesTaxAmount is provided.
• Added automated tests: integration CarLoanControllerTests and unit tests in CalculationServiceTests covering normal and zero-APR scenarios.

Testing

• Ran unit tests in CompoundInterestCalculatorTests including new CalculationServiceTests cases for CalculateCarLoanEstimate, and they passed.
• Ran integration tests including CarLoanControllerTests that post to /api/v1/car-loan/estimate using WebApplicationFactory, and they passed.
• Full test suite executed successfully with no failing tests.

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Include negative trade-in equity in financed balance

The trade-in payoff is currently clamped to a non-negative credit (Math.Max(...)), which drops negative equity when tradeInPayoff > tradeInValue and underestimates amountFinanced, monthly payment, and total paid. In that scenario, the deficit should increase the financed amount (or reduce upfront credit), otherwise requests like a $5,000 trade-in with an $8,000 payoff miss $3,000 of debt in the calculation.

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