Both were exempt from coverage, which is how the 0.38.0 blob-encoding bug
reached a release: nothing exercised the chain that produced it.
Move the historical `documents` shapes into tests/store/legacy_documents.py
so each migration's tests can seed the table as its predecessor left it.
v0.25.0 compresses through compress_json, which switched from gzip to
zstd in 0.38.0 — the same release that added this migration. v0.38.0
decompressed with gzip and fell back to a raw UTF-8 decode, and both
fail on a zstd frame, so upgrading any database older than 0.25.0 has
died with UnicodeDecodeError since the migration shipped.
Reported with a fix by @omaer0 in #586.