Improve Unit Test Coverage for FCM

[vishalchangrani]

Context

The FlowCreditMarket protocol has achieved strong test coverage (89.7%) across core functionality including position management, interest mechanics, liquidations, and rebalancing. However, significant gaps exist in multi-collateral scenarios, oracle failure handling, advanced liquidation cases, and adversarial attack vectors.

This task addresses these high-priority gaps by implementing comprehensive tests for multi-position scenarios, cross-asset operations, oracle resilience, and edge cases to substantially improve protocol robustness and production readiness.

Scope

Close critical testing gaps across 9 high-priority categories:

1. Multi-Position Scenarios - Multiple concurrent positions, batch liquidations, system stress (100+ positions)
2. Multi-Collateral & Cross-Asset - Multi-asset positions, cross-asset borrowing, complex conversions
3. Oracle Failure & Manipulation - Price feed failures, extreme volatility, manipulation resistance
4. Liquidation Edge Cases - Partial liquidations, multi-collateral seizure, bad debt handling
5. Interest Rate Boundaries - Extreme utilization, zero balance cases, long-term accrual
6. Deposit Capacity Attacks - Griefing, front-running, Sybil attacks
7. Rebalancing Failures - Insufficient funds, malicious sources/sinks, gas limits
8. Access Control - Unauthorized access, privilege escalation, entitlement enforcement
9. DeFi Connector Integration - Source/sink failures, reentrancy, malicious connectors

Implementing these test enhancements will increase confidence in the protocol's ability to handle edge cases, resist attacks, and maintain solvency under extreme market conditions.

See #129 for current coverage.