test(invoice-escrow): add 20 edge-case integration tests for issue #352

[rahimatonize]

Changes
File Modified:
integration_test.rs
Tests Added: 20 new integration test functions (prefixed with test_edge_*)
Lines Added: ~508 lines of test code
Test Coverage

1. Boundary Conditions (6 tests)
   ✅ test_edge_minimum_amount_one_stroop - Tests minimum valid amount (1 stroop) through full lifecycle
   ✅ test_edge_large_amounts_near_max - Tests large amounts near i128::MAX/2 to verify no overflow
   ✅ test_edge_due_date_at_current_timestamp_rejected - Verifies due date validation at exact boundary
   ✅ test_edge_refund_at_exact_due_date_allowed - Tests refund at exact due date timestamp
   ✅ test_edge_fee_100_percent - Verifies 100% platform fee (max valid fee)
   ✅ test_edge_tiny_partial_payments - Tests payment in 1-stroop increments
2. Error Code Verification (8 tests)
   ✅ test_edge_fund_zero_amount_rejected - Validates ZeroAmount error
   ✅ test_edge_negative_amount_rejected - Validates InvalidAmount error for negative values
   ✅ test_edge_initialize_fee_over_max_rejected - Validates InvalidFeeBps error
   ✅ test_edge_wrong_payer_rejected - Validates InvalidPayer error
   ✅ test_edge_overpayment_rejected - Validates InvalidAmount error for excess payment
   ✅ test_edge_over_funding_rejected - Validates InvalidAmount error for excess funding
   ✅ test_edge_payment_on_cancelled_escrow_rejected - Validates AlreadySettled error
   ✅ test_edge_multiple_invalid_transitions - Validates multiple error codes in sequence
3. State Persistence (4 tests)
   ✅ test_edge_state_persists_after_failed_fund - Verifies state unchanged after failed funding attempt
   ✅ test_edge_state_persists_after_failed_payment - Verifies state unchanged after failed payment attempt
   ✅ test_edge_state_after_failed_cancel - Verifies state preserved after failed cancellation
   ✅ test_edge_commitment_immutable_lifecycle - Verifies commitment hash never changes throughout lifecycle
4. Concurrent Operations (2 tests)
   ✅ test_edge_concurrent_multiple_fundings - Simulates 3 buyers funding same escrow sequentially
   ✅ test_edge_discounted_invoice - Tests purchase_price < face_value scenario with proper distribution
   Testing Methodology
   Each test follows the established patterns in the codebase:

Uses the setup() helper for consistent test environment initialization
Employs mock_all_auths() for authorization simulation
Verifies both happy-path and error scenarios
Asserts on error codes using try_* methods and assert_eq!(result, Err(Ok(Error::*)))
Validates state persistence by comparing before/after snapshots
Tests token balance distribution after state transitions
Acceptance Criteria Met
✅ At least 10 new integration-style test functions - Added 20 tests
✅ Boundary conditions coverage - 6 tests covering min/max amounts, timestamps, and fees
✅ All error code paths verified - 8 tests explicitly validating error codes
✅ State persistence after failed transactions - 4 tests verifying state integrity
✅ Concurrent operation simulation - 2 tests simulating multiple participants
✅ Follows existing conventions - All tests use established patterns and naming
Verification
Run tests with:

cargo test -p invoice-escrow
All existing tests continue to pass, and the 20 new edge-case tests provide comprehensive coverage of failure scenarios and boundary conditions.
Closes #352

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