[test-isolation] Outbox tests state-sensitive on shared DB

[Aparnap2]

Found during #54 verification: TestOutbox_ClaimMarkRoundTrip + TestOutbox_CrossTenantInvisible fail when outbox_events holds leftover rows from prior sessions (ClaimUnpublished LIMIT 10 saturates with junk). Not a code bug — test isolation gap. Fix: per-test unique tenants (already unique IDs but shared table scan) or table truncation guard. Do NOT fix inside orchestration work. Infrastructure/test track.