Add property tests for the billing fee engine's decimal money arithmetic in services/billing/feeEngine.ts

[barnabasolutayo-lgtm]

##closes #630

Description

This PR introduces robust, deterministic property-based testing using fast-check to assert the algebraic invariants of the billing fee engine (src/services/billing/feeEngine.ts). By replacing simple example-based testing with randomized inputs spanning edge cases, we ensure mathematical correctness and prevent IEEE-754 precision drift or carry bugs in our financial logic.

Invariants Asserted

1. applyFees Invariant (fee + net == gross): Asserts that baseAmount.amount + feeAmount.amount === totalAmount.amount across all inputs.
2. addMoneyStrings Algebraic Properties (tested via public proxy calculateFee and decimalMath helpers):
   • Commutativity: add(a, b) == add(b, a)
   • Associativity: add(add(a, b), c) == add(a, add(b, c))
3. Monotonicity: Asserts that calculateFee is monotonic non-decreasing in the amount for any fixed rate.
4. Parsing & Error Handling:
   • Asserts that parseNonNegativeDecimal rejects negative numbers, NaN values, and malformed structures.
   • Asserts that valid decimal strings successfully round-trip under clean base-100 calculations.
5. No Loss of Precision: Asserts that if the exact intermediate product base * rate / 100 is exactly representable within the target currency's configured scale, the returned fee matches it exactly.

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Verification Results

Automated Test Runs

Tests were verified using vitest:

npx vitest run src/services/billing/feeEngine.property.test.ts src/services/billing/feeEngine.test.ts


Verifies fee+net==gross, add commutativity/associativity, and monotonicity across generated decimal inputs in services/billing/feeEngine.ts.

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