test(refund-vault): add refund window boundary edge-case tests

[Orah-dev]

Summary

Adds three targeted boundary edge-case tests for the RefundVault refund window to guard against off-by-one errors in the > expiry check.

Why

The contract checks current_ledger > paid_at_ledger + window to enforce the refund window. A future refactor changing > to >=, or removing the check entirely, would silently break the boundary semantics. These tests pin the exact ledger at which the window opens and closes so such regressions are caught immediately.

Changes

New tests

• test_refund_exact_boundary_accepted — refund at exactly paid_at + window (the last valid ledger) succeeds. Changing > to >= would break this test.
• test_refund_one_past_boundary_rejected — refund at paid_at + window + 1 (the first invalid ledger) returns WindowExpired. Removing the check entirely would break this test.
• test_zero_window_allows_refund_after_large_advance — when window == 0, the expiry check is bypassed entirely; a refund 10,000 ledgers later still succeeds.

Bugfix

• Fixed missing closing ); and } in test_process_batch_exceeds_max_size_fails.

Contract logic under test

if window > 0 {
    let current_ledger = env.ledger().sequence();
    if current_ledger > paid_at_ledger + window {
        return Err(Error::WindowExpired);
    }
}

Testing

• All new tests are deterministic and use explicit ledger sequence manipulation.
• Each test is documented with the invariant it protects.

Closes #84

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[mergekeeper]

Needs review

Linked to #84, but the diff does not match the issue scope.

The pull request includes an out-of-scope bugfix in CHANGELOG.md and tests that are unrelated to issue #84.

Reviewed commit: b9737e46e797ea706b6b3e1a8ed49074127ae984.

[mergekeeper]

MergeKeeper review

Scope: in scope for linked issue #84.
Verdict: clean

The pull request correctly implements all requested refund window boundary unit tests and includes minor related cleanups without modifying contract logic.

Reviewed commit: 07d51e11689f634f84bd3225fc2a18a47f2f1db1.
CI and merge eligibility are checked separately.

[mergekeeper]

MergeKeeper merge status

Status: blocked
PR state: open
Mergeability: unknown
Checked commit: 07d51e11689f634f84bd3225fc2a18a47f2f1db1.

Reason: One or more required CI checks failed.

Failing checks:

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Next steps:

1. Open the failing check details above and fix the reported error.
2. Run the same checks locally where possible.
3. Commit and push the fix.
4. MergeKeeper will automatically re-review the updated PR.