test(stream): economic-solvency property + fuzzing harness

[Heazzy500]

Closes #1455

Summary

Adds economic-solvency property tests and a contract-driven fuzzing harness to stream_contract, and repairs the pre-existing breakage that prevented the test suite from compiling or running.

Pre-existing state (as found)

• src/property_tests.rs already existed with a proptest! model test (randomized_stream_invariants) at ProptestConfig { cases: 10_000 }, checking withdrawn monotonicity, withdrawn <= deposited, and deposited >= withdrawn + claimable, plus proptest-regressions/property_tests.txt.
• The crate did not compile, so none of it ever ran. Beyond the anticipated acceptance_tests.rs staleness (it targets create_stream_with_cliff, batch_create_streams, transfer_recipient, extend_stream_ttl, types::BatchStreamInput), the library itself had 15 errors: missing StreamError variants (ArithmeticOverflow, StreamStillActive, StreamNotActive), the new Stream::cliff_time field not initialised in four constructors, a Vec<Val> construction bug in create_allowance_stream, and a stale test-only field-count helper.
• storage.rs had STREAM_FIELD_COUNT = 16 while Stream has 17 fields, so try_load_stream matched nothing and every load silently failed. Fixed to 17.

The acceptance-tests gate, the field-count fix, and these minimal compile repairs are the only non-test production edits; they were required for any test to run at all.

What was added

• Pure model fuzz raised to 20,000 cases.
• Cliff protection (contract-driven): create_hybrid_cliff_stream + ledger advance, asserting claimable(t) == 0 for every t < cliff_time and exactly cliff_unlock at the cliff boundary.
• Zero-sum completion: a fully drained stream ends with deposited == withdrawn, claimable == 0, and no stranded dust left in the contract.
• Contract-driven solvency: registers StreamContract, mints a Stellar asset, and runs randomized action sequences (create_stream / create_hybrid_cliff_stream, top_up_stream, withdraw, pause_stream / resume_stream, cancel_stream, ledger advance), asserting after every mutation that token_balance(contract) >= Σ_live (deposited - withdrawn) and that get_claimable_amount >= 0.
• claimable >= 0 under adversarial inputs up to ~1e18, with overflow-safe checked/saturating math.

The case-count split is documented in the module: 20k for the cheap pure model, 512 each for the host-heavy contract-driven tests.

Verification

cargo test -p stream_contract (run from contracts/): 225 passed; 0 failed; 1 ignored (~125s).

One pre-existing test (test_fee_transfer_observes_persisted_stream_on_create_and_top_up) is #[ignore]d with a note: it is built on a same-contract re-entrant call that soroban-env-host 22.x rejects with Contract re-entry is not allowed, so it cannot pass on this host.

cargo fmt is clean on all changed files.

[Heazzy500]

Kindly review and close the issue pls

[Heazzy500]

@Caneryy @LawalRahman
Pls kindly review it and close the issue

[Heazzy500]

@LabsCrypt I merged the latest main into this PR and resolved the conflicts, keeping the property-test changes. The PR is mergeable now; CI is rerunning on the updated head. Please review and merge when the checks are green.

[Heazzy500]

@Caneryy @LawalRahman I synced this PR with current main and resolved the conflicts. I removed the CI-reported unused import and kept a single fee-token test implementation after merging the updated test fixture. Local cargo fmt --all -- --check and cargo clippy --workspace --all-targets -- -D warnings now pass. The updated CI run is starting; please review and merge once all required checks are green.