fix: update key hashing in sparse merkle proof contract

[VGau]

This PR implements issue(s) #

Checklist

• I wrote new tests for my new core changes.
• I have successfully ran tests, style checker and build against my new changes locally.
• I have informed the team of any breaking changes if there are any.

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Note

Medium Risk
Touches cryptographic key hashing used in sparse Merkle proofs; a mismatch would break proof verification and state inclusion checks. Changes are small and covered by added test vectors, but correctness is critical.

Overview
Updates SparseMerkleProof to add hashAccountKey(address) that hashes an account key via Poseidon2 using gnark-compatible padding/word packing, ensuring the on-chain key hash matches external proof generation.

Extends the Hardhat test suite with new account-key test vectors and assertions, and refactors existing hash/leaf tests to use shared expected constants rather than hardcoded hashes.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.62%. Comparing base (b1ab230) to head (772968c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2472   +/-   ##
=========================================
  Coverage     55.62%   55.62%           
  Complexity     1479     1479           
=========================================
  Files           444      444           
  Lines         17820    17820           
  Branches       1954     1954           
=========================================
  Hits           9912     9912           
  Misses         7290     7290           
  Partials        618      618           

Flag	Coverage Δ		*Carryforward flag
hardhat	96.74% <ø> (ø)
kotlin	50.88% <ø> (ø)		Carriedforward from 258eef5

*This pull request uses carry forward flags. Click here to find out more.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.