After the check step returns isValid=true + no errors, a row can still
be silently dropped by /import (response returns activities=[] on
200 OK). Root-cause is usually a field that check hydrates but
/import re-normalises differently (date string form, asset_id resolution).
When we send N valid rows and get back 0, raise ImportValidationError
with a snippet of the check output + first warning — gives the
operator a concrete hint to fix the producer instead of silently
growing dedup against activities that never landed.
poetry run pytest -q → 109 passed, 1 skipped
poetry run mypy → clean
poetry run ruff check → clean
4e2da87637