a two-tuple from encode_prompt is not always (embeds, negative)

Krea 2 returns (prompt_embeds, prompt_embeds_mask), and the generic path read
any 2-tuple as SD 1.x's (embeds, negative_embeds) -- so it handed the mask
over as a negative prompt and the pipeline answered

    If `prompt_embeds` are provided, `prompt_embeds_mask` also have to be
    passed.

which reads as a missing argument rather than a mislabelled one, and arrives
wrapped in "Failed to load image model" two and a half minutes into the load.

Told apart by the pipeline's own __call__ signature rather than a list of
family names, and the answer for those families is to pre-encode nothing:
weighted 75-token windows are a CLIP-pair feature that means nothing to them,
which is why Z-Image already takes that path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZnLhydX9QqUcfBbqLu