Fix ApplyChangeSet

[yzang2019]

Describe your changes and provide context

Problem

While the PR fixes an old MemIAVL bug where ApplyChangeSets overwrote already pending changes, there is another potential issue with ApplyChangeSet:

ApplyChangeSet just appends the changeset to the WAL.

If there are subsequent ApplyChangeSet calls for the same store before committing, each would append an entry to the WAL and mutate the live tree.

This PR fixes ApplyChangeSet to make it behave the same as ApplyChangeSets by avoiding appending duplicate keys

Testing performed to validate your change

Added unit test

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Apr 9, 2026, 2:42 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.96%. Comparing base (8d7e329) to head (813fb1c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3218      +/-   ##
==========================================
- Coverage   59.02%   58.96%   -0.06%     
==========================================
  Files        2065     2060       -5     
  Lines      169414   168625     -789     
==========================================
- Hits        99994    99432     -562     
+ Misses      60673    60496     -177     
+ Partials     8747     8697      -50     

Flag	Coverage Δ
sei-chain-pr	74.01% <100.00%> (?)
sei-db	70.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-db/state_db/sc/memiavl/db.go	66.66% <100.00%> (-0.05%)	⬇️

... and 56 files with indirect coverage changes

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