Unit Test Suite Generator with Edge Cases

You are a Principal Software Engineer in Test (SDET) and TDD master.

Generate a comprehensive, production-grade unit test suite for the following code:

```
export class PricingCalculator {
  calculateTotal(items: CartItem[], userTier: UserTier, couponCode?: string): CalculationResult {
    if (items.length === 0) throw new EmptyCartError("Cannot calculate pricing for empty cart");
    
    let subtotal = items.reduce((sum, item) => sum + (item.price * item.quantity), 0);
    let discount = 0;
    
    if (userTier === 'VIP') discount += subtotal * 0.15;
    if (couponCode === 'SAVE20' && subtotal >= 100) discount += 20;
    
    const taxableAmount = Math.max(0, subtotal - discount);
    const tax = taxableAmount * 0.08;
    return { subtotal, discount, tax, total: taxableAmount + tax };
  }
}
```

Testing Framework: Vitest (TypeScript)
Mocking Library: Vitest vi.fn() and vi.spyOn()
Coverage & Quality Goals: 100% branch/statement coverage, defensive edge case verification.

Produce a test suite adhering to these standards:
1. **Arrange-Act-Assert (AAA) Pattern**:
   - Ensure every test is clean, readable, and isolates a single behavior.
   - Use descriptive test names (`should_<expectedBehavior>_when_<condition>`).
2. **Boundary & Malicious Input Matrix**:
   - Empty collections, null/undefined inputs, zero, negative numbers, max integer bounds.
   - Malformed strings, Unicode characters, and unexpected whitespace.
3. **Mock Isolation**:
   - Mock external I/O (databases, network calls, file systems) strictly at domain boundaries.
   - Avoid over-mocking internal domain logic or value objects.
4. **Failure & Exception Branches**:
   - Test every exception thrown, verifying both exception type and message content.

Output the complete, runnable test file code with clear comments.