The previous test only checked wire decode + fresh shipper state, never
calling shipper.Barrier() against a legacy response source.
New test runs a fake TCP control server that responds with a 1-byte
BarrierOK (no FlushedLSN). Shipper.Barrier() is called against it and
must return an error containing "no FlushedLSN". Verifies the real
rejection path at wal_shipper.go:229-231.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>