Extend fuzz testing beyond the relay contract (escrow, threshold_window)

[Meshmulla]

Summary

We already fuzz the relay contract (soroban/tests/relay_contract_fuzz.rs), which is great — but the other critical contracts have no fuzz coverage. Extending fuzzing to escrow and threshold logic would surface edge cases (overflow, unexpected state transitions, boundary inputs) that unit tests miss.

What we'd like

• Add fuzz targets for at least escrow_contract and threshold_window
• Focus on invariants: balances never go negative, state transitions are always valid, no panics on arbitrary input
• Wire the fuzz targets so they can run in CI for a bounded number of iterations

Where to look

• soroban/tests/relay_contract_fuzz.rs — the existing pattern to follow
• soroban/src/escrow_contract.rs, soroban/src/threshold_window.rs — targets
• soroban/test_snapshots/ — how snapshots are used, so fuzz runs stay deterministic where needed

Acceptance criteria

• Fuzz targets exist for escrow and threshold contracts
• Each asserts clear invariants (no negative balances, no invalid transitions, no panics)
• A short CI job runs them for a capped iteration count
• Any bugs found are filed as separate issues

Notes

Mirror the structure of the relay fuzz test so contributors have a consistent template.

[priscaenoch]

Rather than duplicating the existing relay fuzz tests, I'd identify the invariants that define correct behavior for the escrow and threshold contracts and build the fuzz targets around those guarantees.

That makes the tests resilient to implementation changes while continuing to validate the properties that actually matter.

I'd also keep the fuzz harness lightweight enough to run in CI with a bounded iteration count, while still making it easy to execute longer campaigns locally. That provides continuous coverage in pull requests and a practical workflow for uncovering deeper edge cases without slowing down development.

[grantfox-oss]

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Next steps:

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