eliminate temporary active balances allocation

[ashleychandy]

Description

Remove the temporary active_balances allocation in BankAccountWithCache::load and iterate directly over active balances while preserving the existing behavior.

Changes

• Replaced the temporary Vec<&Balance> with an active balance counter.
• Compute banks_only using the active balance count.
• Iterate directly over balances.iter().filter(|balance| balance.is_active()).
• Eliminated the unnecessary heap allocation while keeping the processing order and validation logic unchanged.