Update sum-tree hashing to match the new spec

[martti007]

No description provided.

[Copilot]

Pull Request Overview

This PR updates the sum-tree hashing implementation to align with a new specification, simplifying the API and improving code maintainability.

Key Changes:

• Refactored SparseMerkleSumTreeRootNode to directly expose hash and value methods instead of returning a Root wrapper object
• Simplified SparseMerkleSumTreePathStep by removing the nested Branch class and flattening the structure with direct data and value fields
• Updated hashing algorithm in FinalizedNodeBranch to directly encode left/right hashes and counters instead of using an intermediate children hash

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
SparseMerkleSumTreeTest.java	Updated test assertions to use new simplified API methods (getValue(), getRootHash(), getData())
TokenSplitBuilder.java	Updated to access root value and hash through new direct methods instead of nested object access
SplitMintReason.java	Simplified coin tree verification by accessing value directly; removed unused Branch import
MintTransactionReasonJson.java	Cleaned up unused imports related to removed serialization classes
SparseMerkleSumTreeRootNode.java	Major refactoring: replaced Root object with direct hash/value access, updated path generation logic to use flattened structure
SparseMerkleSumTreePathStepJson.java	Deleted - JSON serialization now handled by simpler structure
SparseMerkleSumTreePathStepBranchJson.java	Deleted - Branch class removed from API
SparseMerkleSumTreePathStep.java	Simplified by removing Branch inner class and using direct fields for data and value
SparseMerkleSumTreePath.java	Replaced Root field with direct DataHash; updated verification algorithm to work with new structure
FinalizedNodeBranch.java	Updated hashing to encode left/right branches directly instead of computing intermediate children hash
SparseMerkleTreeRootNode.java	Removed unused path field that was always set to the same value
UnicitySealHashMatchesWithRootHashRule.java	Reordered imports for consistency

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[MastaP]

Super minor neatpick, usually type definition is an interface or "var": List<SparseMerkleSumTreePathStep> list = new ArrayList<> or var list = new ArrayList<>. the latter is ok since target is java11