Add property-based contribution tests

[Johnyyy1]

Summary

• adds proptest as a dev dependency for the SoroSave contract tests
• adds randomized contribution-cycle coverage for 2-20 members and variable contribution amounts
• asserts payout order uniqueness, per-round contribution totals, completion status, final member balances, and zero remaining contract balance

Closes #33

Verification

• git diff --check
• Not run: cargo test / cargo fmt / cargo clippy because this local environment does not have a Rust toolchain (cargo and rustfmt are unavailable). The repo CI should run the Rust checks on the PR.