feat(contracts): add property-based tests with proptest (closes #692)

[Mikolo37]

Summary

Implements #692 — property-based tests for the StellarStream Soroban contract using proptest. Four invariants are checked against 10,000 randomly generated stream configurations each, with zero violations.

Property	Invariant
vested_amount_always_in_bounds	0 <= vested_amount(stream, t) <= total_amount for every t
claimable_never_exceeds_vested_minus_claimed	claimable(stream, t) == max(0, vested − claimed); in particular claimable <= vested − claimed whenever vested >= claimed
status_is_always_one_of_four	derived status is always one of Active / Paused / Completed / Canceled and consistent with stored state
cancel_always_produces_canceled_status	after cancel, stream.canceled == true and status == Canceled

Random inputs cover amounts up to 10¹⁸, timestamps up to 2×10⁶, non-zero cliffs, pre-start / mid-stream / post-end times, paused streams, and canceled streams (bounded so the vesting arithmetic cannot overflow i128).

How to run

cd contracts
cargo test --features proptest

The tests live in contracts/src/tests/property_tests.rs (new) and are gated behind a new proptest feature so normal cargo test runs are unaffected. The properties and the on-chain status model are documented in docs/CONTRACT_ABI.md.

Build repairs included (the contract crate did not compile on main)

While getting the property tests running I had to repair the contract crate, which was unbuildable on main due to a series of bad merges. These are required for "all tests pass":

• contracts/src/lib.rs:
  • Removed duplicate use soroban_sdk::{…} imports (E0252).
  • Moved the legacy EscrowVestingContract into the test module — its claim entry point collided with StellarStreamContract::claim (E0428). It is test-only code and not part of the deployed ABI.
  • Declared the missing mod errors; (E0432).
  • Declared testutils + proptest features (fixes unexpected cfg warnings under -D warnings).
  • Added #[allow(clippy::too_many_arguments)] to the ABI-fixed create_stream/create_split_stream signatures.
• contracts/src/test.rs:
  • Reconstructed the test section that was scrambled by the multi-token-allowlist merge — ~30 test bodies were shifted onto the wrong function headers and the file had an unclosed delimiter. Restored the correct bodies for the #593 allowlist tests and the #594 lifecycle tests from the pre-merge branches.
  • Updated tests to the SDK 21 API (mock_all_auths, register_stellar_asset_contract_v2) and to the structured event schema (actor/timestamp/claimed_amount/refunded_amount).
  • Fixed test_create_stream_rejected_after_token_removed_from_allowlist (it could never panic with a single-token allowlist under the lenient test-mode check).
• Snapshots updated for the new event fields; the crate is now rustfmt-clean with the pinned nightly-2024-12-01 toolchain.

Verification

All checks pass on the pinned toolchain (nightly-2024-12-01):

• cargo test — 114 passed, 0 failed
• cargo test --features proptest — 118 passed, 0 failed (4 property tests × 10,000 cases, no violations)
• cargo clippy --all-targets --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• cargo build --target wasm32-unknown-unknown --release — succeeds
• RUSTFLAGS="-D warnings" cargo test --locked — clean

Closes #692

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