use new merkle tree if our root doesn't match the header

[bfish713]

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[gemini-code-assist]

Summary of Changes

Hello @bfish713, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the robustness of epoch reward calculations by implementing a mechanism to reconcile the reward Merkle tree. It ensures that the Merkle tree used for processing rewards is always aligned with the parent block's committed state, preventing potential inconsistencies that could arise from a mismatch. This change improves the reliability and correctness of the reward distribution process.

Highlights

• Merkle Tree Consistency: Introduced logic to ensure the RewardMerkleTreeV2 used for epoch reward calculations is consistent with the parent block's header. If the current state's Merkle tree root does not match the parent header's root, a new RewardMerkleTreeV2 is initialized from the parent's commitment.
• Function Signature Update: The handle_epoch_rewards function in Header now accepts a parent_header: &Header argument, providing necessary context for the Merkle tree consistency check.
• Dependency Update: The ForgetableMerkleTreeScheme trait has been imported, indicating potential future use or a requirement for the updated Merkle tree handling.

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Code Review

The pull request introduces necessary logic to correctly initialize the RewardMerkleTreeV2 for epoch reward calculations. This ensures that the background tasks for reward calculation use the appropriate Merkle tree, either by cloning the current validated state's tree or by reconstructing it from the parent header's commitment if a discrepancy is detected. This change is crucial for maintaining correctness during version transitions and handling potential inconsistencies in the reward state. The modifications are well-placed and address the stated objective of using the new Merkle tree if the root doesn't match the header.