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_

先阅读 `.ai/references/testing.md`，并比较 #14563 中引用的 Krea 2 和 Z-Image 测试文件里的类似注释。在其中记录关于 `output_type="pt"` 的指导，并确保最终的说明涵盖本 issue 中描述的重复 pipeline 测试用例。

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