fix(flux): enable true CFG with precomputed negative embeds

[akshan-main]

What does this PR do?

Addresses the flux review (#13584), Issue 2: FluxImg2ImgPipeline, FluxInpaintPipeline, and FluxControlNetPipeline gated true CFG on true_cfg_scale > 1 and negative_prompt is not None, so passing precomputed negative_prompt_embeds / negative_pooled_prompt_embeds (with negative_prompt=None) silently did not enable CFG.

Base FluxPipeline (and FluxKontextPipeline) already handle this correctly:

has_neg_prompt = negative_prompt is not None or (
    negative_prompt_embeds is not None and negative_pooled_prompt_embeds is not None
)
do_true_cfg = true_cfg_scale > 1 and has_neg_prompt

This brings the three pipelines in line. The downstream if do_true_cfg: block already forwards the precomputed embeds through encode_prompt, so only the gate needed fixing. Verified the gate flips False -> True for the precomputed-embeds case and copies/ruff stay clean.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? (Part of [meta issue] Systematic model/pipeline review findings / tracking #13656, finding from flux model/pipeline review #13584) and discussed on slack with Sayak
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@hlky @sayakpaul @yiyixuxu

[github-actions]

Hi @akshan-main, 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.