Review observations for COPY ON_ERROR_TABLE patch

Hello hackers,

I was reviewing the COPY ON_ERROR_TABLE patch and had a few implementation
questions that may be worth considering.

   - The COPY multi-insert path currently depends on CopyMultiInsertBuffer
   and table_multi_insert() batching behavior. Recovering from row-level
   failures while buffers are partially populated may complicate buffer
   consistency, trigger visibility, or index handling.
   - Would it make sense to initially disable multi-insert batching when
   ON_ERROR_TABLE is enabled (forcing CIM_SINGLE)? That seems like a simpler
   starting point for correctness and recovery semantics.
   - I was also curious about the intended transaction behavior for
   rejected rows. Should rows written to the error table rollback together
   with the surrounding COPY transaction if a later failure occurs?
   - Another possible edge case is recursive failure handling if insertion
   into the error table itself fails.

I have not yet reproduced a concrete failure case, so these are currently
review observations rather than confirmed issues.

Thanks for working on this feature.

Regards,
Vellaipandiyan