Fix redundant Z-Image terminal timestep

[rootonchair]

What does this PR do?

Summary

This PR fixes a redundant terminal denoising step in Z-Image pipelines.

Previously, Z-Image pipelines mutated the scheduler with scheduler.sigma_min = 0.0.
With FlowMatchEulerDiscreteScheduler, this caused the generated timestep/sigma schedule to include a model-forward step at terminal sigma 0.0, while the scheduler also appended its own terminal 0.0. The final denoising step therefore became a no-op transition from 0.0 -> 0.0.

This change makes Z-Image pipelines compute their default sigma schedule up front and pass it through the existing sigmas argument, instead of mutating scheduler.sigma_min.

Behavior

Old Schedule

Requested steps	Scheduler sigmas	Effective updates
9	[1.0, 0.875, ..., 0.125, 0.0, 0.0]	8 meaningful + 1 no-op

New Schedule

Requested steps	Scheduler sigmas	Effective updates
8	[1.0, 0.875, ..., 0.125, 0.0]	8 meaningful

Result Check

Generated 1024x1024 images with Tongyi-MAI/Z-Image-Turbo, prompt dance monkey, seed 0.

New 8 steps

Old 9 steps

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? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@sayakpaul