The backfiller's "no message fetched twice" claim (full-history + resume tests)
previously rested on the insertNew fake de-duping by id, so a re-fetched page was
silently absorbed and `cached.size == TOTAL` could not fail on it. Count the rows
offered to insertNew BEFORE de-dupe and assert it equals TOTAL - WINDOW, so any
re-request of an already-cached page now fails the test. This isolates the real
boundary-descent guarantee and, unlike a fetchOlderThan call-count, is independent
of BACKFILL_BATCH_SIZE.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
JMR-devandClaude Opus 4.8
a73b6410a2