test: add unit tests for rocksdb and pebbledb

[songgaoye]

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PR checklist

• Tests written/updated
• Changelog entry added in .changelog (we use unclog to manage our changelog)

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Note

Adds focused tests to harden DB backends.

• Extend backend_test.go to verify range Compact(start,end) works and preserves data, plus pebble-specific cleanup wait
• New TestPebbleDBNewPebbleDB and TestRocksDBNewRocksDB to assert databases cannot be opened twice for writing
• Add BenchmarkRocksDBRandomReadsWrites; keep existing Pebble benchmark and backend type assertions

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.29%. Comparing base (fb4f703) to head (d1d0bfa).
⚠️ Report is 77 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
+ Coverage   76.80%   79.29%   +2.48%     
==========================================
  Files          23       23              
  Lines        2048     1700     -348     
==========================================
- Hits         1573     1348     -225     
+ Misses        403      282     -121     
+ Partials       72       70       -2     

see 22 files with indirect coverage changes
see 22 files with indirect coverage changes

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