fix: pruning goroutine lifecycle management to prevent nil pointer on Close

[blindchaser]

Describe your changes and provide context

• Lifecycle management: Wrap StateStore with PrunableStateStore that stops the pruning goroutine before closing the underlying database
• Remove duplicate Close: Remove redundant stateStore.Close() from HandleClose() since cms.Close() already handles it
• Defensive checks: Add nil checks in Prune() methods for pebbledb/rocksdb as an additional safety layer
• Idempotency: Use sync.Once to ensure Start/Stop/Close operations are safe to call multiple times

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 4, 2026, 7:07 PM

[codecov]

Codecov Report

❌ Patch coverage is 77.08333% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.65%. Comparing base (fe44b5d) to head (ddd89d9).

Files with missing lines	Patch %	Lines
sei-db/state_db/ss/pruning/manager.go	79.41%	5 Missing and 2 partials ⚠️
sei-db/db_engine/pebbledb/mvcc/db.go	0.00%	1 Missing and 1 partial ⚠️
sei-db/db_engine/rocksdb/mvcc/db.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2800      +/-   ##
==========================================
+ Coverage   46.94%   56.65%   +9.70%     
==========================================
  Files        1965     2031      +66     
  Lines      160525   165884    +5359     
==========================================
+ Hits        75363    93980   +18617     
+ Misses      78637    63666   -14971     
- Partials     6525     8238    +1713     

Flag	Coverage Δ
sei-chain	41.51% <80.43%> (-0.01%)	⬇️
sei-cosmos	48.13% <ø> (+<0.01%)	⬆️
sei-db	68.42% <0.00%> (-0.31%)	⬇️
sei-tendermint	58.05% <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	71.46% <ø> (+0.21%)	⬆️
sei-db/state_db/ss/store.go	53.22% <100.00%> (+7.94%)	⬆️
sei-db/db_engine/pebbledb/mvcc/db.go	63.85% <0.00%> (-0.23%)	⬇️
sei-db/db_engine/rocksdb/mvcc/db.go	57.31% <0.00%> (-0.47%)	⬇️
sei-db/state_db/ss/pruning/manager.go	83.72% <79.41%> (+42.81%)	⬆️

... and 320 files with indirect coverage changes

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