Fix QwenImageEditPlus max_sequence_length and transformer text seq len kwargs

[tonera]

What does this PR do?

This PR fixes two issues in QwenImageEditPlusPipeline (pipeline_qwenimage_edit_plus.py) related to text sequence length handling:

• Make max_sequence_length effective: the parameter was accepted by the public API but previously not applied. We now truncate prompt_embeds and prompt_embeds_mask to max_sequence_length.

• Prevent failures in the denoising loop: QwenImage transformer/RoPE needs an explicit text sequence length, but some accelerated/quantized transformer implementations (e.g. nunchaku) don’t accept the new max_txt_seq_len kwarg. The pipeline now adapts to the transformer forward signature:

• Prefer passing max_txt_seq_len when supported (or when **kwargs is accepted)

• Otherwise fall back to deprecated txt_seq_lens

• Otherwise pass nothing (let the implementation handle it)

• This avoids runtime errors such as:

• ValueError: Either max_txt_seq_len or txt_seq_lens ... must be provided

• TypeError: ... forward() got an unexpected keyword argument 'max_txt_seq_len'

• Who can review?

Tagging: @yiyixuxu @asomoza (pipelines)