Escrow Settlement with Exact Due Date Timestamp - Tests

Task Progress

Test 1: test_settlement_at_exact_due_date — Settle at exact due date
Test 2: test_settlement_after_due_date_before_refund — Settle after due date (before refund)
Test 3: test_settlement_at_exact_due_date_with_partial_payment — Partial payment at exact due date
Test 4: test_settlement_at_exact_due_date_state_persistence — State persistence verification
Test 5: test_refund_prevented_after_settlement_at_exact_due_date — Refund blocked after settlement at due date
Test 6: test_settlement_at_exact_due_date_emits_correct_events — Event emissions verification
Run cargo test --all to verify all tests pass
Run cargo fmt and cargo clippy for style compliance