fix: [M-01] struct encoding

[filmakarov]

fixes M-01

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PR-Codex overview

This PR focuses on optimizing the encoding of data for struct hashing in the HashLib contract and related tests. It simplifies the process by directly using itemHashes instead of an intermediate encodedData variable.

Detailed summary

• In HashLib.sol, encodedData variable is removed; structHash now uses itemHashes directly.
• In MeeK1Validator_Simple_Mode_Test.t.sol, encodedData variable is removed; structHash now uses stxItemHashes directly.

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