refactor(l1,l2): extract duplicate witness generation logic to helper

[Itodo-S]

Motivation

Both functions shared a significant amount of identical code for reconstructing partial tries and fetching block headers. This refactor reduces maintenance burden and improves code readability.

Description

Extracted the duplicated witness generation logic from [generate_witness_for_blocks_with_fee_configs] and [generate_witness_from_account_updates] into a new helper function [construct_execution_witness]

Changes

• Created [construct_execution_witness] helper function in [crates/blockchain/blockchain.rs]
• Updated [generate_witness_for_blocks_with_fee_configs] and [generate_witness_from_account_updates] to use the helper.

Verification

• Ran cargo test -p ethrex-blockchain to ensure no regressions were introduced.
• All tests passed.

Closes #5708