Fix gas reserve validation for high-decimal tokens in withdraw operations

[Copilot]

What was the issue?

The withdraw validation in validateGasReserve() was ineffective for high-decimal tokens. When operationAmount is undefined (withdraw all scenario), the validation checked balance < gasReserve where gasReserve = max(balance * 1%, 1n). This made the check mathematically impossible to fail for any non-zero balance, allowing withdrawals that would leave insufficient tokens for gas payment.

What was fixed?

Gas reserve calculation for high-decimal tokens:

• Changed calculateGasReserve() to return max(percentageReserve, fixedMinimum) for tokens with >6 decimals
• fixedMinimum uses parseUnits(GAS_RESERVE_MINIMUM, decimals) (currently 0.01 tokens)
• Ensures balance-independent minimum that makes withdraw validation meaningful

TypeScript signature fix:

• Changed withdraw() signature from amount?: string to amount: string | undefined
• Resolves build error where optional parameter preceded required parameter

Before:

// High-decimal token with 1 token balance
// gasReserve = max(1 token * 1%, 1n) = 0.01 tokens
// Check: 1 < 0.01 → false (validation ineffective)

After:

// gasReserve = max(0.01 tokens, 0.01 tokens) = 0.01 tokens  
// Check: 1 < 0.01 → false, but 0.005 < 0.01 → true (validation effective)

Related issues

Addresses feedback from #49

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.