document output_type="pt" in the agent docs #14616

If I remember correctly a lot of migrated test files add a similar comment (for example, the Krea 2 and Z-Image tests also have similar comments in #14563), so I think we should consider adding a note in .ai/references/testing.md about this instead of having comments in each pipeline test file.

Originally posted by @dg845 in #14599 (comment)