Implement property-based testing suite for the SDK using fast-check

[Kingsman-99]

Overview

Add property-based tests using fast-check that verify SDK invariants hold for arbitrary inputs. This goes beyond unit tests by generating hundreds of random inputs per property and shrinking failures to minimal reproducible cases.

Requirements

• Install fast-check as a dev dependency
• Write property tests for:
  • parseAmount / formatAmount roundtrip: formatAmount(parseAmount(x)) === x for all valid USDC strings
  • isValidAddress: any 56-char G-prefixed base32 string passes; anything else fails
  • deadlineFromDays(n) > Date.now() / 1000 for all n > 0
  • createInvoice parameters: arbitrary recipient counts (1–20), amounts summing to any positive value
  • StellarSplitClient methods: sequential call sequences never leave the client in an inconsistent state (using a mock contract)
• Each property must run at least 500 examples
• All properties must have meaningful fc.assert failure messages for debugging

Acceptance Criteria

• fast-check installed and configured
• At least 5 property test files covering the above
• Each property runs 500+ examples
• All properties pass
• npm test passes with zero failures
• TypeScript compiles with zero errors
• ESLint passes with zero errors

Definition of Done

All CI checks must pass before the PR is reviewed.

[nurudeenheroic]

I've gone through the issue and am ready to get started.

[zeemscript]

Hello @maintainer, I’ve gone through the requirements and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me.

[levoski1]

"I'd be happy to work on this issue. I have the required expertise and can start immediately if assigned."

[grantfox-oss]

🦊 GrantFox — @levoski1 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #418)
2. Your PR will be reviewed by the Stellar-split maintainers

Good luck! Track your progress on GrantFox.