fix(flux): decouple modular flux from classic and qwenimage imports

[akshan-main]

What does this PR do?

Addresses Issue 6 of the flux review (#13584). The modular Flux blocks imported FluxPipeline._pack_latents and _prepare_latent_image_ids, the classic Kontext PREFERRED_KONTEXT_RESOLUTIONS constant, and two QwenImage modular helpers. This replaces those cross-imports with flux-modular-local copies, following decoders.py.

The coupling check from the issue returns nothing and the modular flux tests pass.

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) Discussed on slack with Sayak
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? (refactor, covered by the existing modular flux tests)

Who can review?

@DN6 @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.

[sayakpaul]

Sorry I don't get it. What's the problem with the existing imports? These assumed that the modular pipelines were implemented w.r.t their standard counterparts. @yiyixuxu am I missing something?