Fix Rocksdb MVCC read timestamp lifetime for iterators

[masih]

RocksDB ReadOptions timestamp handling in MVCC reads had unsafe lifetime semantics:

• newTSReadOptions used a local [8]byte timestamp and passed it to SetTimestamp.
• Under concurrent iterator creation, the timestamp backing memory could be reused, causing versioned reads to drift to the wrong version.
• Iterator read options were not owned/cleaned up with iterator lifetime, making behavior fragile and cleanup inconsistent.

The changes here use heap-backed timestamp slices
(make([]byte, TimestampSize)) when calling SetTimestamp / SetIterStartTimestamp, and create ReadOptions once per iterator in Iterator and ReverseIterator, passing them into the iterator wrapper. The mvcc.iterator is extended to own *grocksdb.ReadOptions and destroy them in Close(), while getSlice is updated to destroy per-read ReadOptions after GetCF. RawIterate is also updated to pass owned read options into the iterator lifecycle. As a minor cleanup, Next() is made a pointer receiver for consistency.

This should also fix the flaky rocksdb tests, flaked most recently on main among other PRs with unrelated changes.

[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 24, 2026, 7:22 PM

[codecov]

Codecov Report

❌ Patch coverage is 88.52459% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.68%. Comparing base (1b1e9a2) to head (1b881dd).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-db/db_engine/rocksdb/mvcc/db.go	63.15%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2971       +/-   ##
===========================================
+ Coverage   58.14%   73.68%   +15.54%     
===========================================
  Files        2107       13     -2094     
  Lines      173322     2440   -170882     
===========================================
- Hits       100780     1798    -98982     
+ Misses      63602      522    -63080     
+ Partials     8940      120     -8820     

Flag	Coverage Δ
sei-chain	74.68% <100.00%> (+16.56%)	⬆️
sei-db	69.50% <85.10%> (+1.01%)	⬆️

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/pebbledb/mvcc/iterator.go	56.01% <100.00%> (+0.41%)	⬆️
sei-db/db_engine/rocksdb/mvcc/iterator.go	86.86% <100.00%> (+1.31%)	⬆️
sei-db/state_db/ss/test/storage_test_suite.go	99.29% <100.00%> (ø)
sei-db/db_engine/rocksdb/mvcc/db.go	58.40% <63.15%> (+0.91%)	⬆️

... and 2095 files with indirect coverage changes

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