chore(api): document Responses not-found errors (#3959)

## Summary

Document the JSON error envelope returned when a requested response or
input-item cursor cannot be found.

## Changes

- Describe wrapped 404 errors for response retrieval, deletion,
cancellation, and input-item listing.

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>