[CONTRACT] Add comprehensive cross-contract integration test suite

[dinahmaccodes]

Tier: 🔴 Hard

Description:

Create integration tests that verify correct behavior when interacting with mocked token contracts, oracle services, and payment routing contracts. Tests should cover failure modes and edge cases in inter-contract communication.

Tasks:

• Create mock token contract stubs
• Implement mock oracle for external data
• Create integration test scenarios
• Test failure handling in cross-contract calls
• Add tests for token bridge scenarios

File locations:

• Test file → tests/test_cross_contract_integration.rs
• Mock contracts → tests/mocks/ directory

Acceptance Criteria:

• All critical cross-contract paths tested
• Failure scenarios handled gracefully
• Token transfer failures captured
• Oracle failures handled with fallback
• Tests document expected contract interactions

PR Checklist:

• Integration test suite implemented
• Mock contracts functional and realistic
• Failure scenarios covered
• Tests verify error handling
• cargo test shows 100% pass rate

Reference: Read CONTRIBUTING.md before starting.