chore: remove redundant slice allocations in DiffKVStores

[Doryu001]

Drop the preallocation of kvAs/kvBs slices that were immediately overwritten by getKVPairs retain existing concurrency logic while trimming an unnecessary heap allocation per iterator

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.27%. Comparing base (84e841a) to head (03abbbf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25538      +/-   ##
==========================================
+ Coverage   69.99%   70.27%   +0.28%     
==========================================
  Files         827      822       -5     
  Lines       54070    53627     -443     
==========================================
- Hits        37844    37689     -155     
+ Misses      16226    15938     -288     

Files with missing lines	Coverage Δ
testutil/sims/simulation_helpers.go	56.19% <100.00%> (ø)

... and 8 files with indirect coverage changes

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[aljo242]

low impace