Validate load-balancing policy name early in ManagedChannelBuilder.defaultLoadBalancingPolicy()

Modify `defaultLoadBalancingPolicy(String)` to validate the provided policy name against the LoadBalancerRegistry immediately when the setter is called. If the policy name is not found in the registry, the method should throw an exception. This change ensures that configuration errors, such as typos, are caught at the call site rather than during channel construction.