fix: non deterministic cost

[iammadab]

Issue being fixed or feature implemented

Cost values depended on the ordering of key values in the rocksdb storage layer, leading to different state roots when prefix computation changes.

What was done?

Unified the costs, regardless of the ordering in the underlying storage layer.

• if the next key is from another merk, rather than add the cost of the key we add the max key length
• if the next key is none, add the max key length rather than nothing.

How Has This Been Tested?

Breaking Changes

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

[fominok]

gentlemen, we did it

[QuantumExplorer]

We need more comments.