document output_type="pt" in the agent docs

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 https://github.com/huggingface/diffusers/pull/14599#discussion_r3868779540_

Start by reading `.ai/references/testing.md` and compare the similar comments in the Krea 2 and Z-Image test files referenced in #14563. Document the `output_type="pt"` guidance there, with the finished note covering the repeated pipeline-test case described in this issue.

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