Exhaustive Edge-Case Unit & Integration Test Generator

You are an Elite QA Architect and Test Automation Specialist. I need an exhaustive, production-grade test suite covering both the happy paths and obscure edge cases for the following code. Testing Framework / Language: Vitest + TypeScript Target Function or Service Code: export function calculateProratedRefund( billingCycleStart: Date, billingCycleEnd: Date, cancellationDate: Date, monthlyFeeCents: number ): number { const totalCycleMs = billingCycleEnd.getTime() - billingCycleStart.getTime(); const usedMs = cancellationDate.getTime() - billingCycleStart.getTime(); const unusedRatio = Math.max(0, 1 - (usedMs / totalCycleMs)); return Math.round(monthlyFeeCents * unusedRatio); } Known business requirements or constraints: 1. Refund must never exceed monthlyFeeCents; 2. If canceled after cycleEnd, refund must be 0; 3. If canceled on exact cycleStart, full refund; 4. Gracefully handle invalid dates or cycleEnd <= cycleStart without throwing unhandled exceptions. Deliver a comprehensive test suite: 1. Edge-Case Matrix: A table listing every identified edge condition categorized by: (A) Boundary Values (off-by-one, 0, MAX_INT), (B) Temporal/Timezone traps, (C) Null/Undefined/Type Coercion, (D) Concurrency / Idempotent retries. 2. Complete Test Suite Code: Self-contained, runnable test suite using idiomatic Vitest + TypeScript conventions with clear arrange-act-assert structure. 3. Descriptive Test Names: Follow the `describe('method') -> it('should [expected behavior] when [specific condition]')` standard. 4. Mocking & Isolation Strategy: Minimal, clean mocks for external network/database dependencies without over-mocking internal logic.