Three small follow-ups from #2866 self-review:
1. TestIntegration_Sweeper_StaleHeartbeatIsMarkedStuck — assert
strings.Contains(errDet, "no heartbeat for") instead of != "".
The original "non-empty" check passes for any error_detail value;
if a future regression swaps the message format, the test wouldn't
catch it. Pin the production format string explicitly.
2. TestIntegration_Sweeper_DeadlineExceededIsMarkedFailed — drop the
redundant `last_heartbeat = now()` write. The sweeper checks
deadline FIRST (the stronger statement) and short-circuits before
evaluating heartbeat staleness, so the heartbeat field is irrelevant
for that test path.
3. integrationDB doc comment now warns explicitly that the helper is
NOT t.Parallel()-safe — it hot-swaps the package-level mdb.DB and
restores via t.Cleanup. If a future contributor adds t.Parallel()
to one of these tests they race on the global. Comment makes the
constraint discoverable instead of a debugging surprise.
All 7 integration tests still pass against real Postgres locally.
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