Fix: Add 5% fee margin buffer for scheduling estimation variance

[kgrgpg]

Summary

Adds a 5% margin buffer to the fee estimation in AutoBalancer.scheduleNextRebalance() to handle variance in scheduling cost calculations.

Problem

When AutoBalancers self-schedule recurring rebalances, they estimate the required fees via FlowTransactionScheduler.estimate(). However, the actual fee at scheduling time can sometimes slightly exceed the estimate, causing the scheduling to fail with insufficient fees even when the account has adequate funds.

Solution

Add a 5% buffer (1.05 multiplier) when:

1. Checking if sufficient fees are available
2. Withdrawing fees for scheduling

// Check with 5% margin buffer to match withdrawal
if config.txnFunder.minimumAvailable() < (estimate.flowFee! * 1.05) {
    return "INSUFFICIENT_FEES_AVAILABLE"
}

// withdraw the fees from the funder with a margin buffer
let feeWithMargin = estimate.flowFee! * 1.05
let fees <- config.txnFunder.withdrawAvailable(maxAmount: feeWithMargin) as! @FlowToken.Vault

Changes

• DeFiActions.cdc: Added 5% fee margin buffer in scheduleNextRebalance()
• Improved nil-safety for error handling
• Minor comment improvements

Testing

Tested in FlowVaults-sc with comprehensive scheduling tests - all 18 tests pass.