chore: avoid double parsing tx_summary in offline execution

[VolodymyrBg]

execute_imported_proposal was re-parsing tx_summary from JSON even though ExportedProposal::to_proposal had already deserialized it. This led to unnecessary work and allocations and was inconsistent with the online execution flow, which relies on proposal.tx_summary directly. Micro-optimization and a bit of clean-up

[github-actions]

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[VolodymyrBg]

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[VolodymyrBg]

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