fix: remove unbounded collect in finalize_all_subnet_root_dividends

[ionodeionode]

Summary

Fixes #2411

Problem

finalize_all_subnet_root_dividends collects all hotkey keys from RootClaimable storage into a Vec before iterating and mutating each one:

let hotkeys = RootClaimable::<T>::iter_keys().collect::<Vec<_>>();

This is O(N) in both reads and memory where N is the total number of hotkeys with any root claimable balance. Since this function is called from do_dissolve_network (an extrinsic), with thousands of hotkeys this could:

• Exceed block weight limits
• Cause high memory usage from the collect::<Vec<_>>()

Solution

Replace the unbounded `collect()" with a lazy iterator drain:

let mut cursor = RootClaimable::<T>::iter_keys();
while let Some(hotkey) = cursor.next() {
    RootClaimable::<T>::mutate(&hotkey, |claimable| {
        claimable.remove(&netuid);
    });
}

This processes each hotkey without materializing the full key set in memory. Substrate's storage iterators are safe to use while mutating other keys of the same map (cursor invalidation only occurs when the current key is removed, which we don't do here — we only mutate the value).

Impact

• Memory: O(1) instead of O(N)
• Behavior: Functionally identical — same keys are processed, same mutations applied
• Weight: The actual computation remains the same, but avoids the risk of exceeding block limits due to memory allocation