test(escrow-fuzz): cover refund_escrow and circuit-breaker interaction

[Mac-5]

closes #31

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Context

#5 added fuzz coverage for escrow_contract's settlement path (create_escrow, release_escrow, batch_release) and wired it into the bounded fuzz CI job. That coverage is solid but has a real gap: refund_escrow — the other half of the settlement path, where value also moves — has zero fuzz coverage, and the circuit breaker added afterward in 053a3f8 (which guards release_escrow, batch_release, and refund_escrow) was never fuzzed at all, since it landed after #5 merged. This PR closes that specific gap. It does not re-litigate #5's scope — it's a follow-up, not a fix for something broken.

What needs to be done (from the original ask, re-verified against the current gap)

• Add a fuzz target for escrow settlement/release, mirroring the existing pattern →
  Extends escrow_contract/tests/escrow_contract_fuzz.rs (doesn't add a new file — the target already exists per Extend fuzz testing beyond the relay contract (escrow, threshold_window) #5; this adds one more #[test] to it, following the same seeded-xorshift-RNG / try_* client / capped-iteration conventions as the other three tests in the file).
• Drive randomized sequences of operations, not single calls →
  fuzz_release_refund_sequences_respect_circuit_breaker_and_never_double_pay randomly interleaves sync_verification, release_escrow, refund_escrow, trip_circuit_breaker, and reset_circuit_breaker against the same escrow per round — the "record usage/lock, then settle/release" sequencing the issue asked for, applied to the one path (refund_escrow + breaker) the existing suite didn't touch.
• Invariants asserted after every single op:
  • released_amount never negative, never exceeds the releasable total (no residue/overdraw)
  • an escrow can never reach both Released and Refunded (no double payout)
  • Released status implies released_amount exactly equals the releasable total (no residue left behind)
  • a tripped breaker blocks both release_escrow and refund_escrow with CircuitBreakerTripped specifically, never a different error or a silent success
  • no operation ever returns a host-level panic/error for in-range input (Ok(Err())/Err(Err()) → hard panic! in the test, treated as a bug)
• Wire into the bounded Fuzz CI job with a capped iteration count →
  No CI change needed — .github/workflows/ci.yml's existing cargo test --test escrow_contract_fuzz -p escrow-contract line already picks up the new #[test] automatically. Round count is capped at fuzz_iterations().min(20), independent of the job's FUZZ_ITERATIONS=200, mirroring the same cap already used by threshold_window_fuzz::fuzz_max_windows_cap_and_removal_keep_state_consistent.

Acceptance criteria

• A fuzz target exercises the escrow settlement/release path with clearly stated invariants
• It runs in the bounded Fuzz CI job with a capped iteration count
• Any bug it uncovers is filed separately with a minimal reproduction — N/A: no bug found. Every invariant held across a 2,000-round local stress run (FUZZ_ITERATIONS=2000 cargo test --release, see Evidence).
• CI checks must be green before the PR can be merged — verified locally on this exact diff (see Evidence)

Evidence

• cargo test --test escrow_contract_fuzz -p escrow-contract (all 4 tests, including the new one): ok, 4 passed in 0.9s
• Exact CI invocation, FUZZ_ITERATIONS=200 cargo test --test escrow_contract_fuzz -p escrow-contract: ok, 4 passed in 1.56s
• FUZZ_ITERATIONS=2000 cargo test --release --test escrow_contract_fuzz -p escrow-contract -- --exact fuzz_release_refund_sequences_respect_circuit_breaker_and_never_double_pay: ok, 1 passed in 47s, no invariant violation
• cargo fmt --all -- --check: clean
• cargo clippy --all-targets --all-features -- -D warnings (whole workspace): clean
• cargo test (whole workspace): ok, 553+ tests passed, 0 failed
• cargo build --release --target wasm32-unknown-unknown: succeeds

Known gaps

• emergency_recover (admin-only recovery while paused) also moves released_amount but isn't fuzzed here — it's a distinct admin-emergency path, not part of the normal settlement flow this issue targeted.
• challenge_escrow/resolve_challenge (the multisig dispute path) isn't exercised together with release/refund/breaker in the same randomized sequence — combining all four adds significant complexity; scoping it out keeps this PR reviewable.
• A heavier/nightly sweep beyond the capped 20-round default is explicitly out of scope per the original issue's own notes ("a heavier nightly/scheduled sweep can be added later as its own piece of work").
• Unrelated, pre-existing, not fixed here: the circuit-breaker tests added in 053a3f8 (escrow_contract/tests/escrow_contract.test.rs) never had their test_snapshots/*.json fixtures committed. Discovered during this audit; flagging rather than fixing since it's outside this PR's scope.

Scope

Single file touched: escrow_contract/tests/escrow_contract_fuzz.rs, plus its auto-generated test_snapshots/*.json fixtures (21 files, one per capped round + one for the deterministic tail — same convention as the pre-existing fuzz_max_windows_cap_and_removal_keep_state_consistent test). No contract logic changed; no CI file changed.

[Meshmulla]

Good codes. Thanks