feat(arbitrage): Add TradeFailed event with detailed failure diagnostics

[Neros0]

Description

This PR enhances observability and debugging capabilities within TestArbitrage by introducing a new TradeFailed event. This complements existing trade lifecycle events (TradeInitiated, TradeCompleted) by explicitly surfacing failure scenarios with structured context.

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✅ What’s New

🛑 TradeFailed Event Added

A fully Natspec-documented event that captures granular failure insights:

/// @notice Emitted when a trade fails with detailed error information
/// @param tradeId Unique identifier for this trade
/// @param reason Error message describing the failure
/// @param step Which step of the process failed (1=first swap, 2=second swap, 3=repayment)
/// @param gasUsed Gas consumed before failure
event TradeFailed(uint256 indexed tradeId, string reason, uint8 step, uint256 gasUsed);