Implement flash loan + liquidation combo integration tests

[Smartdevs17]

Context

Flash loans combined with liquidations are complex attack vectors and legitimate use cases. Integrated testing ensures both paths work correctly.

Current Limitation/Problem

Flash loans and liquidations are only tested in isolation. Their interaction may reveal unexpected behavior.

Expected Outcome

Integration tests that verify flash loan + liquidation combos work correctly and without exploitable interleaving.

Acceptance Criteria

• Test: flash loan → borrow → liquidate → repay flash loan (profitable)
• Test: flash loan → manipulate price → liquidate → repay (circuit breaker stops)
• Test: multiple flash loans in same transaction cascade
• Test: flash loan liquidation when position is at boundary health factor
• Test: liquidation with flash-loaned funds across multiple positions
• Verify: protocol fee is correctly collected in flash-loan liquidations
• Verify: liquidation cannot be griefed via flash loan sandwich
• Gas benchmark: flash loan liquidation vs standard liquidation
• Edge cases: partial liquidation with flash loan, empty flash loan

Technical Scope

• Files: tests/integration/flash-loan-liquidation/
• Contracts: flash-loan, liquidation-engine, lending-pool
• Test scenarios: profitable, unprofitable, boundary conditions