fix(api): correct the eval run cancellation endpoint (#4003)

## Summary

Fixes eval run cancellation requests to use the cancellation endpoint.

## Changes

- Send eval run cancellation requests to
/evals/{eval_id}/runs/{run_id}/cancel.
- Preserve the existing cancellation method parameters and response
types.

Co-authored-by: marcuswood-oai <323319819+marcuswood-oai@users.noreply.github.com>