refactor(sei-db): encapsulate ReadOptions lifecycle in iterator constructors

[pdrobnjak]

Stacked on #2929.

Summary

• Pushes ReadOptions creation into the iterator constructors so callers never handle ReadOptions directly
• newIterator() creates a versioned iterator with internally managed ReadOptions, used by Iterator() and ReverseIterator()
• newRangeIterator() creates a timestamp-range iterator with internally managed ReadOptions, used by RawIterate()
• Adds Timestamp() method on the iterator wrapper so RawIterate can access per-entry timestamps without reaching into the raw grocksdb.Iterator
• Unexports the iterator constructor (NewRocksDBIterator -> buildIterator)

Test plan

• No behavioral changes — purely structural refactor
• All RocksDB tests should pass

🤖 Generated with Claude Code

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Feb 19, 2026, 10:16 AM

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.37%. Comparing base (73de9e4) to head (769e9be).

Files with missing lines	Patch %	Lines
sei-db/db_engine/rocksdb/mvcc/iterator.go	29.41%	12 Missing ⚠️
sei-db/db_engine/rocksdb/mvcc/db.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##           pd/rocksdb-readopts-lifecycle    #2931      +/-   ##
=================================================================
+ Coverage                          57.34%   57.37%   +0.02%     
=================================================================
  Files                               2095     2095              
  Lines                             172896   172900       +4     
=================================================================
+ Hits                               99145    99196      +51     
+ Misses                             64868    64823      -45     
+ Partials                            8883     8881       -2     

Flag	Coverage Δ
sei-chain	52.87% <ø> (+0.03%)	⬆️
sei-cosmos	48.19% <ø> (ø)
sei-db	68.58% <33.33%> (-0.60%)	⬇️

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

Files with missing lines	Coverage Δ
sei-db/db_engine/rocksdb/mvcc/db.go	59.49% <50.00%> (+1.26%)	⬆️
sei-db/db_engine/rocksdb/mvcc/iterator.go	77.47% <29.41%> (-8.84%)	⬇️

... and 33 files with indirect coverage changes

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