Fix type safety and error handling in parallel balance check

[Copilot]

Addresses code review feedback on the parallel balance check optimization for SquidRouter phase handler.

Type Safety

• quote.outputCurrency: as any → as FiatToken
• balanceCheckPromise: Promise<any> → Promise<Big>
• Added isEvmNetwork() type guard to validate destination before EVM balance checks

Error Handling

• Wrapped Promise.any() inputs with .catch() to prevent unhandled rejections when the losing promise rejects after winner resolves
• Rejected promises now use Error objects instead of plain strings
• Distinguish BalanceCheckError types in failure messages:
  • Timeout: Expected behavior when bridge is slow
  • ReadFailure: Unexpected infrastructure issue

// Before: Both errors treated identically
throw new Error(`Both checks failed. Errors: ${errors.join(", ")}`);

// After: Meaningful distinction
if (balanceError.type === BalanceCheckErrorType.Timeout) {
  errorMessage += ` Balance check timed out after ${timeout}ms.`;
} else if (balanceError.type === BalanceCheckErrorType.ReadFailure) {
  errorMessage += ` Balance check read failure (unexpected infrastructure issue): ${msg}.`;
}

Documentation

• Documented 15-minute timeout rationale (vs 3-5 minutes in other handlers): cross-chain settlement and gas payment legitimately take longer under network congestion

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