test: add property-based fuzz coverage for Zod validators

- Add fuzz tests for all Zod schemas in invoices.ts, bids.ts, settlements.ts, shared.ts
- Use fast-check to generate malformed payloads: unicode, very large ints, NaN/Infinity
- Test prototype-pollution attempts (__proto__, constructor) cannot mutate output
- Assert validators never throw unhandled errors on any input
- Covers all exported schemas with 100% test pass rate (31/31 tests pass)
- Security-checklist.md already has comprehensive fuzz strategy documentation (Section 12)

Closes #2356

Author: Yasir Abdulsalam