Add negative_prompt to Flux2KleinPipeline

[akshan-main]

What does this PR do?

Fixes #13416

Adds negative_prompt parameter to Flux2KleinPipeline.__call__(). Previously, the unconditional prompt was hardcoded to "" when CFG was active, with no way for users to provide a custom negative prompt.

Changes:

• Added negative_prompt: str | list[str] | None = None parameter
• Falls back to "" when None (backward compatible)
• Added length validation when both prompt and negative_prompt are lists
• Fixed negative_prompt_embeds type hint from str | list[str] to torch.Tensor
• Updated docstrings

Fixes #13416

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? how to use zimage and flux2 with negative prompt? #13416
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@asomoza @sayakpaul