fix: DefaultPendingChildKeyCooldown should use prod_or_fast value

[DanielDerefaka]

Summary

DefaultPendingChildKeyCooldown was returning 0 instead of using the same value as DefaultPendingCooldown. This causes PendingChildKeyCooldown to be 0 on fresh chains (like localnet) instead of the expected 7200 (prod) or 15 (fast/test).

Changes

• Changed DefaultPendingChildKeyCooldown to call DefaultPendingCooldown::<T>::get() for consistency

Before

pub fn DefaultPendingChildKeyCooldown<T: Config>() -> u64 {
    0
}

After

pub fn DefaultPendingChildKeyCooldown<T: Config>() -> u64 {
    DefaultPendingCooldown::<T>::get()
}

Fixes #2107