fix: prevent nil pointer panic during pruning on shutdown

[blindchaser]

Describe your changes and provide context

• Remove redundant stateStore.Close() from HandleClose() since cms.Close() already handles it via rootmulti.Store.Close() -> ssStore.Close()
• Add defensive nil checks in pebbledb/rocksdb Prune() methods to gracefully return error instead of panic if db is already closed

Testing performed to validate your change

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Feb 6, 2026, 5:13 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.13%. Comparing base (97d1db2) to head (799d477).
⚠️ Report is 6 commits behind head on release/v6.3.

❌ Your project status has failed because the head coverage (38.27%) is below the target coverage (40.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v6.3    #2801      +/-   ##
================================================
- Coverage         43.36%   42.13%   -1.23%     
================================================
  Files              1866     1465     -401     
  Lines            155305   114756   -40549     
================================================
- Hits              67351    48356   -18995     
+ Misses            81941    62285   -19656     
+ Partials           6013     4115    -1898     

Flag	Coverage Δ
sei-chain	42.55% <ø> (+0.01%)	⬆️
sei-cosmos	38.03% <ø> (+<0.01%)	⬆️
sei-db	69.39% <100.00%> (+24.46%)	⬆️
sei-ibc-go	55.96% <ø> (ø)
sei-tendermint	?
sei-wasmd	41.57% <ø> (ø)
sei-wasmvm	39.88% <ø> (ø)

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

Files with missing lines	Coverage Δ
app/app.go	75.65% <ø> (+0.28%)	⬆️
sei-db/ss/rocksdb/db.go	59.44% <100.00%> (+0.64%)	⬆️

... and 407 files with indirect coverage changes

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[masih]

This should do a compare and swap and proceed if not closed already

[blindchaser]

good point