Fix descent from split check

[michaelsproul]

Issue Addressed

Fix an issue on Holesky after true finalization was reached, whereby blocks on the canonical chain were rejected due to "not descending from the split block".

I think the issue here is that is_descendant will return false for any ancestor_root that has been pruned. Once we got true finality back fork choice got pruned before the split block was updated on some/most nodes, so they hit this error case.

Proposed Changes

Only do the is_descent check if the split.slot is greater than the finalized_slot, or equivalently, set is_descendant_from_split_block when split.slot <= finalized_slot.