fix(diffusers): keep negative_prompt unless negative embeds are set

The sd_embed branch dropped negative_prompt for every pipeline, which
discarded the user's negative prompt for Flux, where no negative embeds
are produced. Only drop it when negative_prompt_embeds is present, and
move the check to a helper that has unit tests.

Assisted-by: Claude Code:claude-sonnet-5-5
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

backend/python/diffusers/load_options.py

@@ -5,3 +5,14 @@ def single_file_load_kwargs(original_config_file: str, from_single_file: bool) -
     if from_single_file and original_config_file:
         return {"original_config_file": original_config_file}
     return {}
+
+
+def drop_negative_prompt_if_embedded(kwargs: dict) -> dict:
+    """Remove negative_prompt from pipeline kwargs when negative_prompt_embeds is set.
+
+    Diffusers rejects both being passed together. Pipelines without negative
+    embeddings keep the user's negative prompt.
+    """
+    if "negative_prompt_embeds" in kwargs:
+        kwargs.pop("negative_prompt", None)
+    return kwargs