fix(monero-harness): add missing ? operator in address_at

[segflaunt]

Summary

The address method in Wallet::address_at was missing the ? operator for error propagation, causing a type mismatch.

Before:

Ok(self.wallet.address(account_index, address_index).await)

After:

Ok(self.wallet.address(account_index, address_index).await?)

Testing

• Code compiles successfully
• monero-harness package builds without errors
• Integration tests require Docker containers (not run locally)

Checklist

• Code formatted with dprint
• Clippy passes (for this package)
• Builds successfully

🤖 Generated with Claude Code

[binarybaron]

Thank you