Fix clip_skip being ignored for negative prompt embeddings in SDXL pipelines

[Dev-X25874]

What does this PR do?

clip_skip was silently ignored when computing negative prompt embeddings
in all three SDXL encode_prompt methods. The positive prompt path
correctly branches on clip_skip:

if clip_skip is None:
    prompt_embeds = prompt_embeds.hidden_states[-2]
else:
    prompt_embeds = prompt_embeds.hidden_states[-(clip_skip + 2)]

but the negative prompt path unconditionally used hidden_states[-2]
regardless of the clip_skip value. This means that whenever clip_skip
is set, positive and negative embeddings are extracted from different hidden
layers, breaking the symmetry that classifier-free guidance relies on.

Fixed by mirroring the existing conditional to the negative path in:

• pipeline_stable_diffusion_xl.py
• pipeline_stable_diffusion_xl_img2img.py
• pipeline_stable_diffusion_xl_inpaint.py

A test is added to test_stable_diffusion_xl.py that calls encode_prompt
with clip_skip=1 vs clip_skip=None and asserts that negative embeddings
differ between the two calls (i.e. clip_skip now actually affects them).

Fixes # (issue)

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Who can review?

@yiyixuxu @asomoza

[github-actions]

Hi @Dev-X25874, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.