Update test_sqlitedb.cpp and test_backend.cpp to match the current
PutAttribute/GetAttribute/GetAll signatures (owner + flags params added
in the Stage 3 attr column split).
Add null-checks on owner/flags out-params in CSQLiteDB::GetAttribute()
and CSQLiteBackend::Get() so callers can safely pass nullptr when they
don't need the metadata.
All 20 tests pass (10 CSQLiteDB + 10 IStorageBackend).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
