fix: list legacy LMDB cache children

[nikooo777]

Summary

Fix legacy LMDB child listing so bundler recovery can discover cached recovery state written without explicit group markers.

Older bundler cache entries write leaf pseudopaths directly, for example:

• ~bundler@1.0/tx/<txid>/status
• ~bundler@1.0/item/<itemid>/bundle

but they do not necessarily write a <<"group">> marker at the parent path. Current hb_cache:list/2 goes through hb_store:read/3, so listing a parent like ~bundler@1.0/tx returns [] when the parent marker is missing, even though child keys exist underneath it.

That causes startup recovery to miss old in-progress bundle states.

What changed

• hb_cache:list/3 now delegates to hb_store:list/3 instead of using hb_store:read/3.
• hb_store_lmdb:list/3 now falls back to listing child keys for a missing parent path, while still rejecting direct listing of simple leaf values.
• Added a regression test for LMDB children that exist without a group marker.

Why

Bundler recovery depends on prefix listing:

• dev_bundler_cache:load_bundle_states/1 lists ~bundler@1.0/tx
• dev_bundler_cache:load_items/2 lists ~bundler@1.0/item

Without this fix, legacy LMDBs with child pseudopaths but no parent group marker look empty to recovery.

Tests

• rebar3 eunit --module=hb_cache
• rebar3 eunit --module=hb_store_lmdb