Function select_server_adaptive 

pub fn select_server_adaptive<'a>(
    servers: &'a [&ProxyServer],
) -> &'a ProxyServer

Adaptive load balancer using UCB (Upper Confidence Bound) algorithm This is a multi-armed bandit approach that balances exploration and exploitation

Advantages over weighted random:

• Automatically explores underutilized servers
• Adapts to changing server performance
• Reduces oscillation by considering uncertainty
• Theoretical guarantees on regret bounds