igapyon import-state-handler
Start in pending. If schema validation fails, transition to rejected and
stop. If validation succeeds but the remote API returns 429, wait for the
Retry-After value and retry at most twice. After the second 429, transition to
deferred. For any other API error, transition to failed without retry. When
all records are accepted, transition to completed and write receipt.json.
Never write receipt.json in rejected, deferred, or failed states. Return
the final state, retry count, and receipt path or absence.